Unregister the NAND device from the NAND subsystem when removing a denali
NAND controller, otherwise the MTD attached to the NAND device is still
exposed by the MTD layer, and accesses to this device will likely crash
the system.

Fixes: 2a0a288e ("mtd: denali: split the generic driver and PCI layer")
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NDinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>