mtd: Parse mtdparts/mtdids again when the MTD list has been updated

Updates to the MTD device list should trigger a new parsing of the
mtdids/mtdparts vars even if those vars haven't changed.

Fixes: 5db66b3a

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