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>

drivers/mtd/nand/raw/sunxi_nand.c

@@ -544,7 +544,7 @@ static void sunxi_nfc_write_buf(struct mtd_info *mtd, const uint8_t *buf,
 
 static uint8_t sunxi_nfc_read_byte(struct mtd_info *mtd)
 {
-	uint8_t ret;
+	uint8_t ret = 0;
 
 	sunxi_nfc_read_buf(mtd, &ret, 1);