mtd: rawnand: rename default ->onfi_get/set_features() implementations

Prepare future work on the ->onfi_get/set_features() hooks by renaming
the core's implementation as 'default' ones.

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