watchdog: cros-ec: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:

drivers/watchdog/cros_ec_wdt.c:29:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/Z-WG6_uhWsy_FCq3@kspp
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Author: GustavoARSilva

drivers/watchdog/cros_ec_wdt.c

@@ -25,26 +25,22 @@ static int cros_ec_wdt_send_cmd(struct cros_ec_device *cros_ec,
 				union cros_ec_wdt_data *arg)
 {
 	int ret;
-	struct {
-		struct cros_ec_command msg;
-		union cros_ec_wdt_data data;
-	} __packed buf = {
-		.msg = {
-			.version = 0,
-			.command = EC_CMD_HANG_DETECT,
-			.insize  = (arg->req.command == EC_HANG_DETECT_CMD_GET_STATUS) ?
-				   sizeof(struct ec_response_hang_detect) :
-				   0,
-			.outsize = sizeof(struct ec_params_hang_detect),
-		},
-		.data.req = arg->req
-	};
-
-	ret = cros_ec_cmd_xfer_status(cros_ec, &buf.msg);
+	DEFINE_RAW_FLEX(struct cros_ec_command, msg, data,
+			sizeof(union cros_ec_wdt_data));
+
+	msg->version = 0;
+	msg->command = EC_CMD_HANG_DETECT;
+	msg->insize  = (arg->req.command == EC_HANG_DETECT_CMD_GET_STATUS) ?
+		   sizeof(struct ec_response_hang_detect) :
+		   0;
+	msg->outsize = sizeof(struct ec_params_hang_detect);
+	*(struct ec_params_hang_detect *)msg->data = arg->req;
+
+	ret = cros_ec_cmd_xfer_status(cros_ec, msg);
 	if (ret < 0)
 		return ret;
 
-	arg->resp = buf.data.resp;
+	arg->resp = *(struct ec_response_hang_detect *)msg->data;
 
 	return 0;
 }