Add ap->qc_active and ap->sactive, mask of all active qcs and libata's
view of the SActive register, respectively.  Also, implement
ata_qc_complete_multiple() which takes new qc_active mask and complete
multiple qcs according to the mask.

These will be used to track NCQ commands and complete them.  The
distinction between ap->qc_active and ap->sactive is also useful for
later PM implementation.
Signed-off-by: NTejun Heo <htejun@gmail.com>