mtd: nand: mxc: Add buffer argument to copy_spare

With following patches we will have to copy the spare data to/from
other buffers, so add the buffer as argument to copy_spare().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>