mtd: rawnand: sunxi: Make sure ret is initialized in sunxi_nfc_read_byte()

Fixes the following smatch warning:

drivers/mtd/nand/raw/sunxi_nand.c:551 sunxi_nfc_read_byte() error: uninitialized symbol 'ret'.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>