ACPI: APEI: EINJ: Fix less than zero comparison on a size_t variable

The check for c < 0 is always false because variable c is a size_t which
is not a signed type. Fix this by making c a ssize_t.

Fixes: 90711f7 ("ACPI: APEI: EINJ: Create debugfs files to enter device id and syndrome")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20250624201032.522168-1-colin.i.king@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: ColinIanKing

drivers/acpi/apei/einj-core.c

@@ -913,7 +913,7 @@ static ssize_t u128_write(struct file *f, const char __user *buf, size_t count,
 	u8 tmp[COMPONENT_LEN];
 	char byte[3] = {};
 	char *s, *e;
-	size_t c;
+	ssize_t c;
 	long val;
 	int i;