libata: fix ata_scsi_change_queue_depth()

Fix ata_scsi_change_queue_depth() such that...

* NCQ on/off is exactly determined using the same logic as the issue path.

* queue depth is adjusted to 1 if NCQ is not enabled.

* -EINVAL is returned if requested action is ignored due to limitations.

This fixes the bug which allows queue depth to be increased on
blacklisted NCQ hosts/devices.

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