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: 9fdd78f7

 ("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