libata: add support for NCQ commands for SG interface

This patch is needed to make NCQ commands with FPDMA protocol value
(eg READ/WRITE FPDMA) work over SCSI Generic (SG) interface.

Signed-off-by: Vinayak Kale <vinayak.kale@seagate.com>
Signed-off-by: Tejun Heo <tj@kernel.org>