mtd: rawnand: atmel: Convert the driver to exec_op()

Both SMC and HSMC are converted to exec_op().

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200720131356.1579073-6-tudor.ambarus@microchip.com