net: mscc: ocelot: reapply bridge forwarding mask on bonding join/leave

Applying the bridge forwarding mask currently is done only on the STP
state changes for any port. But it depends on both STP state changes,
and bonding interface state changes. Export the bit that recalculates
the forwarding mask so that it could be reused, and call it when a port
starts and stops offloading a bonding interface.

Now that the logic is split into a separate function, we can rename "p"
into "port", since the "port" variable was already taken in
ocelot_bridge_stp_state_set. Also, we can rename "i" into "lag", to make
it more clear what is it that we're iterating through.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>