mtd: nand: mxc: reorder functions to avoid forward declarations

We'll call copy_spare() and mxc_do_addr_cycle() from another place
during the next patches, so move functions up to avoid forward
declarations.

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