rtc: s35390a: use u8 instead of char for register buffer

The register buffer in s35390a_get_reg() was previously defined as
`char *buf`. This is not ideal since register data represents raw
binary values rather than textual data.

Switch the type to `u8 *buf` to better reflect its intended usage and
to avoid potential issues with sign extension when handling register
values on platforms where `char` is signed by default.

This change improves type safety and makes the code consistent with
other RTC drivers that operate on raw register data.

Signed-off-by: Nick Huang <sef1548@gmail.com>
Link: https://patch.msgid.link/20250920174224.108795-1-sef1548@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: yaodehaung

drivers/rtc/rtc-s35390a.c

@@ -66,7 +66,7 @@ struct s35390a {
 	int twentyfourhour;
 };
 
-static int s35390a_set_reg(struct s35390a *s35390a, int reg, char *buf, int len)
+static int s35390a_set_reg(struct s35390a *s35390a, int reg, u8  *buf, int len)
 {
 	struct i2c_client *client = s35390a->client[reg];
 	struct i2c_msg msg[] = {
@@ -83,7 +83,7 @@ static int s35390a_set_reg(struct s35390a *s35390a, int reg, char *buf, int len)
 	return 0;
 }
 
-static int s35390a_get_reg(struct s35390a *s35390a, int reg, char *buf, int len)
+static int s35390a_get_reg(struct s35390a *s35390a, int reg, u8 *buf, int len)
 {
 	struct i2c_client *client = s35390a->client[reg];
 	struct i2c_msg msg[] = {
@@ -168,7 +168,7 @@ static int s35390a_read_status(struct s35390a *s35390a, char *status1)
 
 static int s35390a_disable_test_mode(struct s35390a *s35390a)
 {
-	char buf[1];
+	u8 buf[1];
 
 	if (s35390a_get_reg(s35390a, S35390A_CMD_STATUS2, buf, sizeof(buf)) < 0)
 		return -EIO;
@@ -210,7 +210,7 @@ static int s35390a_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	struct i2c_client *client = to_i2c_client(dev);
 	struct s35390a	*s35390a = i2c_get_clientdata(client);
 	int i;
-	char buf[7], status;
+	u8 buf[7], status;
 
 	dev_dbg(&client->dev, "%s: tm is secs=%d, mins=%d, hours=%d mday=%d, "
 		"mon=%d, year=%d, wday=%d\n", __func__, tm->tm_sec,
@@ -239,7 +239,7 @@ static int s35390a_rtc_read_time(struct device *dev, struct rtc_time *tm)
 {
 	struct i2c_client *client = to_i2c_client(dev);
 	struct s35390a *s35390a = i2c_get_clientdata(client);
-	char buf[7], status;
+	u8 buf[7], status;
 	int i, err;
 
 	if (s35390a_read_status(s35390a, &status) == 1)
@@ -273,7 +273,7 @@ static int s35390a_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
 {
 	struct i2c_client *client = to_i2c_client(dev);
 	struct s35390a *s35390a = i2c_get_clientdata(client);
-	char buf[3], sts = 0;
+	u8 buf[3], sts = 0;
 	int err, i;
 
 	dev_dbg(&client->dev, "%s: alm is secs=%d, mins=%d, hours=%d mday=%d, "\
@@ -326,7 +326,7 @@ static int s35390a_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alm)
 {
 	struct i2c_client *client = to_i2c_client(dev);
 	struct s35390a *s35390a = i2c_get_clientdata(client);
-	char buf[3], sts;
+	u8 buf[3], sts;
 	int i, err;
 
 	err = s35390a_get_reg(s35390a, S35390A_CMD_STATUS2, &sts, sizeof(sts));
@@ -383,7 +383,7 @@ static int s35390a_rtc_ioctl(struct device *dev, unsigned int cmd,
 {
 	struct i2c_client *client = to_i2c_client(dev);
 	struct s35390a *s35390a = i2c_get_clientdata(client);
-	char sts;
+	u8 sts;
 	int err;
 
 	switch (cmd) {
@@ -422,7 +422,7 @@ static int s35390a_probe(struct i2c_client *client)
 	unsigned int i;
 	struct s35390a *s35390a;
 	struct rtc_device *rtc;
-	char buf, status1;
+	u8 buf, status1;
 	struct device *dev = &client->dev;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))