mtd: Make sure the name passed in mtdparts fits in mtd_name[]

The local mtd_name[] variable is limited in size. Return an error if
the name passed in mtdparts does not fit in this local var.

Fixes: 5db66b3a

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