mtd: txx9ndfmc: Use nand_release to free resources

This patch fixes memory leak on chip->bbt and chip->buffers.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>