sata_mv: simplify request/response queue handling

Try and simplify handling of the request/response queues.

Maintain the cached copies of queue indexes in a fully-masked state,
rather than having each use of them have to do the masking.

Split off handling of a single crpb response into a separate function,
to reduce complexity in the main mv_process_crpb_entries() routine.

Ignore the rarely-valid error bits from the crpb status field,
as we already handle that information in mv_err_intr().

For now, preserve the rest of the original logic.
A later patch will deal with fixing that separately.

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