net: dsa: don't fast age bridge ports with learning turned off

On topology changes, stations that were dynamically learned on ports
that are no longer part of the active topology must be flushed - this is
described by clause "17.11 Updating learned station location information"
of IEEE 802.1D-2004.

However, when address learning on the bridge port is turned off in the
first place, there is nothing to flush, so skip a potentially expensive
operation.

We can finally do this now since DSA is aware of the learning state of
its bridged ports.

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