sata_mv: PHY_MODE4 cleanups

The handling for PHY_MODE4 was originally just cloned from the
Marvell proprietary driver (with their blessing).
But we can do better than that.

Tidy things up with some judicious mask definitions, to improve maintainability.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>