ata: ATA Command Priority Disabled By Default

Add a sysfs entry to turn on priority information being passed
to a ATA device. By default this feature is turned off.

This patch depends on ata: Enabling ATA Command Priorities

tj: Renamed ncq_prio_on to ncq_prio_enable and removed trivial
    ata_ncq_prio_on() and open-coded the test.

Signed-off-by: Adam Manzanares <adam.manzanares@hgst.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/drivers/ata/libahci.c b/drivers/ata/libahci.c
index 0d028ea..ee7db31 100644
--- a/drivers/ata/libahci.c
+++ b/drivers/ata/libahci.c
@@ -140,6 +140,7 @@
 struct device_attribute *ahci_sdev_attrs[] = {
 	&dev_attr_sw_activity,
 	&dev_attr_unload_heads,
+	&dev_attr_ncq_prio_enable,
 	NULL
 };
 EXPORT_SYMBOL_GPL(ahci_sdev_attrs);