mtd: maps: fix error return code of physmap_flash_remove()

When platform_get_drvdata() returns NULL to info, no error return code
of physmap_flash_remove() is assigned.
To fix this bug, err is assigned with -EINVAL in this case

Fixes: 73566edf ("[MTD] Convert physmap to platform driver")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210308034446.3052-1-baijiaju1990@gmail.com