mtd: nand: xway: fix nand locking

The external Bus Unit (EBU) can control different flash devices, but
these NAND flash commands have to be atomic and should not be
interrupted in between. Lock the EBU from the beginning of the command
till the end by moving the lock to the chip select.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>