Let's use a helper to clearly check if an operation is supported or not.

Return -ENOTSUPP when ->exec_op() is not implemented as we cannot know.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200507105241.14299-8-miquel.raynal@bootlin.com