sm501: Log unimplemented raster operation modes

The sm501 currently implements only a very limited set of raster operation
modes. After this change, unknown raster operation modes are logged so
these can be easily spotted.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>