mtd: denali: remove calling mtd_device_unregister() after nand_release()

The implementation of nand_release() already call mtd_device_unregister(),
no need to call it again.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>