mtd: rawnand: onfi: Use the BIT() macro when possible

Update the onfi.h header to use the BIT() macro.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Dahl <ada@thorsis.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-5-miquel.raynal@bootlin.com