mtd: spi-nor: Fix errno on Quad Enable methods

When the Read-Modify-Write-Read-Back Quad Enable methods failed on
the Read-Back, they returned -EINVAL. Since this is an I/O error,
return -EIO.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>