mtd: Implement mtd_{read,write}() as wrappers around mtd_{read,write}_oob()

mtd_{read,write}_oob() already take care of checking the params and
calling ->_{read,write}() or  ->_{read,write}_oob() based on the
request and the operations supported by the MTD device.
No need to duplicate the logic, we can simply implement
mtd_{read,write}() as wrappers around mtd_{read,write}_oob().

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>