We recently added some new locking to this function but one error path
was overlooked.  We need to drop the lock before returning.
Fixes: f4da56529da6 ("net: stmmac: Add support for external trigger timestamping")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
                acr_value |= PTP_ACR_ATSEN3;
                break;
        default:
+               mutex_unlock(&priv->aux_ts_lock);
                return -EINVAL;
        }
        writel(acr_value, ptpaddr + PTP_ACR);