mtd: rawnand: onfi: Use intermediate variables to improve readability

Before reworking a little bit the ONFI detection code, let's
clean the coding style of the if statements to improve readability.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200428094302.14624-5-miquel.raynal@bootlin.com