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 <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/libata.h

···
 };
 
 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 */