mtd: pxa3xx_nand: fix a memory leak

In pxa3xx_nand_remove, we should call nand_release instead of
mtd_device_unregister to properly free bad block table memory
and bad block descriptor memory.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>