mtd: rawnand: rename SET/GET FEATURES related functions

SET/GET FEATURES are flagged ONFI-compliant because of their name. This
is not accurate as non-ONFI NAND chips support it and use it.

Rename the hooks and helpers to remove the "onfi" prefix.

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