mtd: rawnand: vf610_nfc: support ONFI SET/GET_FEATURES commands

With the move to ->exec_op() the driver should now support ONFI
SET/GET_FEATURES commands.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>