net: dpaa2-switch: phylink_disconnect_phy needs rtnl_lock

There is an ASSERT_RTNL in phylink_disconnect_phy which triggers
whenever dpaa2_switch_port_disconnect_mac is called.

To follow the pattern established by dpaa2_eth_disconnect_mac, take the
rtnl_mutex every time we call dpaa2_switch_port_disconnect_mac.

Fixes: 84cba729

 ("dpaa2-switch: integrate the MAC endpoint support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>