mtd: rawnand: sunxi: Use a consistent name for sunxi_nand_chip objects

sunxi_nand_chip objects are sometimes called chip and other times
called sunxi_nand. Make that consistent and name all occurrences
sunxi_nand.

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