mtd: rawnand: sunxi: Remove an unnecessary check

Each chip is required to have a unique CS number ("reg" property) in the
range 0-7, so there is no need to separately count the number of chips.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221229181526.53766-4-samuel@sholland.org