mtd: Fallback to ->_read/write_oob() when ->_read/write() is missing

mtd: Fallback to ->_read/write_oob() when ->_read/write() is missing

Some MTD sublayers/drivers are implementing ->_read/write_oob() and
provide dummy wrappers for their ->_read/write() implementations.
Let the core handle this case instead of duplicating the logic.

Signed-off-by: Boris Brezillon <[email protected]>
Acked-by: Robert Jarzmik <[email protected]>
Acked-by: Brian Norris <[email protected]>
Reviewed-by: Miquel Raynal <[email protected]>
Tested-by: Ladislav Michl <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Reviewed-by: Jagan Teki <[email protected]>