libata: convert NCQ test in ata_qc_issue() to ata_is_ncq()

I missed one while converting to ata_is_*() protocol test helpers.
Convert it.  Pointed out by Jeff Garzik.

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