mtd: rawnand: sunxi: Remove an unnecessary check

sunxi_nand->nsels cannot be zero, so the second check implies the first.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221229181526.53766-3-samuel@sholland.org