mtd: rawnand: xway: Fix build issue

This MIPS driver does not support COMPILE_TEST yet and failed to build
under my radar.

Replace 'mtd' chich is not defined in the scope of xway_nand_remove()
by nand_to_mtd(chip). The mistake has been added in the long series
dropping nand_release().

Tested with a 7.3.0 MIPS GCC toolchain built with Buildroot.

Fixes: 9fdd78f7bc ("mtd: rawnand: xway: Stop using nand_release()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200626065511.16424-1-miquel.raynal@bootlin.com

drivers/mtd/nand/raw/xway_nand.c

@@ -224,7 +224,7 @@ static int xway_nand_remove(struct platform_device *pdev)
 	struct nand_chip *chip = &data->chip;
 	int ret;
 
-	ret = mtd_device_unregister(mtd);
+	ret = mtd_device_unregister(nand_to_mtd(chip));
 	WARN_ON(ret);
 	nand_cleanup(chip);