mtd: core: clear out unregistered devices a bit more

This allows an MTD device that has been unregistered to be easily
re-registered later without triggering spurious "already registered"
warnings in mtd_device_parse_register() and add_mtd_device().

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211015185049.3318-1-zev@bewilderbeest.net