sata_mv: restructure mv_qc_issue

Rearrange logic in mv_qc_issue() to handle protocols
other than ATA_PROT_DMA, ATA_PROT_NCQ, and ATA_PROT_PIO.
This is in preparation for later enabling ATAPI support.

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