tile: fix missing unlock on error in tile_net_open()

Add the missing unlock before return from function tile_net_open()
in the error handling case.

Introduced by commit f3286a3a.
(tile: support multiple mPIPE shims in tilegx network driver)
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: NChris Metcalf <cmetcalf@tilera.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/tile/tilegx.c

@@ -1510,8 +1510,10 @@ static int tile_net_open(struct net_device *dev)
 
 	/* Get the instance info. */
 	rc = gxio_mpipe_link_instance(dev->name);
-	if (rc < 0 || rc >= NR_MPIPE_MAX)
+	if (rc < 0 || rc >= NR_MPIPE_MAX) {
+		mutex_unlock(&tile_net_devs_for_channel_mutex);
 		return -EIO;
+	}
 
 	priv->instance = rc;
 	instance = rc;