libata: fix ata_scmd_need_defer()

Fix ata_scmd_need_defer() such that...

* whether NCQ is used or not is exactly determined using the same
  criteria as the issue path.

* defer-check is performed in all cases.

This fixes race condition where turning off NCQ on the fly causes
non-NCQ commands sneak into NCQ phase.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>