[libata] sata_mv: Add a new mv_sff_check_status() function to sata_mv.

This is necessary for use with the upcoming "mv_qc_issue_fis()" patch,
but is being added separately here for easier code review.

When using command issue via the "mv_qc_issue_fis()" mechanism,
the initial ATA_BUSY bit does not show in the ATA status (shadow) register.
This can confuse libata!  So here we add a hook to fake ATA_BUSY
for that situation, until the first time a BUSY, DRQ, or ERR bit is seen.

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