mtd: rawnand: Take check_only into account

->exec_op() is passed a check_only argument that encodes when the
controller should just check whether the operation is supported or not
without executing it. Some controllers simply ignore this arguments,
others don't but keep modifying some of the registers before returning.
Let's fix all those drivers.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200418194217.1016060-1-boris.brezillon@collabora.com