net: dsa: unlock the rtnl_mutex when dsa_master_setup() fails

After the blamed commit, dsa_tree_setup_master() may exit without
calling rtnl_unlock(), fix that.

Fixes: c146f9bc ("net: dsa: hold rtnl_mutex when calling dsa_master_{setup,teardown}")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>