mtd: Be more strict on the "mtdparts=" prefix check

strstr() does not guarantee that the string we're searching for is
placed at the beginning. Use strncmp() instead.

Fixes: 5db66b3aee ("cmd: mtd: add 'mtd' command")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Heiko Schocher <hs@denx.de>

drivers/mtd/mtd_uboot.c

@@ -215,7 +215,7 @@ int mtd_probe_devices(void)
 		return 0;
 
 	/* Start the parsing by ignoring the extra 'mtdparts=' prefix, if any */
-	if (strstr(mtdparts, "mtdparts="))
+	if (!strncmp(mtdparts, "mtdparts=", sizeof("mtdparts=") - 1))
 		mtdparts += 9;
 
 	/* For each MTD device in mtdparts */