net: dsa: felix: update bridge fwd mask from ocelot lib when changing tag_8021q CPU

Add more logic to ocelot_port_{,un}set_dsa_8021q_cpu() from the ocelot
switch lib by encapsulating the ocelot_apply_bridge_fwd_mask() call that
felix used to have.

This is necessary because the CPU port change procedure will also need
to do this, and it's good to reduce code duplication by having an entry
point in the ocelot switch lib that does all that is needed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>