stmmac: intel: unlock on error path in intel_crosststamp()

We recently added some new locking to this function but one error path
was overlooked.  We need to drop the lock before returning.

Fixes: f4da56529d ("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>

drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c

@@ -320,6 +320,7 @@ static int intel_crosststamp(ktime_t *device,
 		acr_value |= PTP_ACR_ATSEN3;
 		break;
 	default:
+		mutex_unlock(&priv->aux_ts_lock);
 		return -EINVAL;
 	}
 	writel(acr_value, ptpaddr + PTP_ACR);