ata: define AC_ERR_OK

Since we will return enum ata_completion_errors from qc_prep in the next
patch, let's define AC_ERR_OK to mark the OK status.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/libata.h

@@ -484,6 +484,7 @@ enum hsm_task_states {
 };
 
 enum ata_completion_errors {
+	AC_ERR_OK		= 0,	    /* no error */
 	AC_ERR_DEV		= (1 << 0), /* device reported error */
 	AC_ERR_HSM		= (1 << 1), /* host state machine violation */
 	AC_ERR_TIMEOUT		= (1 << 2), /* timeout */