mtd: spi-nor: core: use EOPNOTSUPP instead of ENOTSUPP

ENOTSUPP is not a SUSV4 error code. Using EOPNOTSUPP is preferred
in its stead.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201005153138.6437-2-p.yadav@ti.com