mtd: rawnand: hynix: Use ->exec_op() in hynix_nand_reg_write_op()

Modern NAND controller drivers implement ->exec_op() instead of
->cmdfunc(), make sure we don't end up with a NULL pointer dereference
when hynix_nand_reg_write_op() is called.

Fixes: 8878b126

 ("mtd: nand: add ->exec_op() implementation")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>