net: dsa: felix: re-enable TX flow control in ocelot_port_flush()

Because flow control is set up statically in ocelot_init_port(), and not
in phylink_mac_link_up(), what happens is that after the blamed commit,
the flow control remains disabled after the port flushing procedure.

Fixes: eb4733d7

 ("net: dsa: felix: implement port flushing on .phylink_mac_link_down")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>