ata: Enabling ATA Command Priorities

This patch checks to see if an ATA device supports NCQ command priorities.
If so and the user has specified an iocontext that indicates
IO_PRIO_CLASS_RT then we build a tf with a high priority command.

This is done to improve the tail latency of commands that are high
priority by passing priority to the device.

tj: Removed trivial ata_ncq_prio_enabled() and open-coded the test.

Signed-off-by: Adam Manzanares <adam.manzanares@hgst.com>
Signed-off-by: Tejun Heo <tj@kernel.org>