sata_sil24: implement PMP support

Implement PMP support.  sil24 supports full FIS-switching.  However,
it has a PMP DMA CS errata which requires port-wide resetting if
commands are outstanding to three or more devices when an error occurs
on one of them.

ATAPI commands often result in CHECK SENSE and it's crucial to not
reset them before fetching sense data.  Unfortunately, ATAPI CHECK
SENSE causes a lot of problem if command is outstanding to any other
device usually resulting in port-wide reset.  So, sata_sil24
implements sil24_qc_defer() which guarantees ATAPI command is run by
itself.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>