sata_mv: simplify freeze/thaw bit-shift calculations

Introduce the MV_PORT_TO_SHIFT_AND_HARDPORT() macro,
to centralize/simplify various scattered bits of logic
for calculating bit shifts and the like.

Some of the places that do this get it wrong, too,
so consolidating the algorithm at one place will help
keep the code correct.

For now, we use the new macro in mv_eh_{freeze,thaw}.
A subsequent patch will re-use this in the interrupt handlers

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