mtd: maps: physmap: Check mtd_device_{parse_register, unregister}() ret code

mtd_device_parse_register() and mtd_device_unregister() can fail,
check their return code and propagate the error to the upper layer
if needed.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>