mtd: Fix error handling in mtd_device_parse_register() error path

Check and log del_mtd_device() failures. Print an error message
with pr_err() to prevent silent failures, but preserve the original
error code instead of propagating the secondary error since
del_mtd_device() is already in an error handling path.

Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>