linux_spi: Use fgets() to read buffer size

Since fread() returns the number of bytes read, this currently will only
check for errors if it returns 0 (i.e. the file was empty). However, it
is possible for fread() to encounter an error after reading a few bytes,
which this doesn't catch. Fix this by using fgets() instead, which will
return NULL if EOF or an error is encountered, and is simpler anyway.

Change-Id: I4f37c70e97149b87c6344e63a57d11ddde7638c4
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1403824
Reviewed-on: https://review.coreboot.org/c/flashrom/+/34848
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67848
Reviewed-by: Felix Singer <felixsinger@posteo.net>

linux_spi.c

@@ -138,8 +138,7 @@ int linux_spi_init(void)
 	}
 
 	char buf[10];
-	memset(buf, 0, sizeof(buf));
-	if (!fread(buf, 1, sizeof(buf) - 1, fp)) {
+	if (!fgets(buf, sizeof(buf), fp)) {
 		if (feof(fp))
 			msg_pwarn("Cannot read %s: file is empty.\n", BUF_SIZE_FROM_SYSFS);
 		else