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: 5db66b3a ("cmd: mtd: add 'mtd' command")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Heiko Schocher <hs@denx.de>