net: dsa: sja1105: exit sja1105_vlan_filtering when called multiple times

VLAN filtering is a global property for sja1105, and that means that we
rely on the DSA core to not call us more than once.

But we need to introduce some per-port state for the tagger, namely the
xmit_tpid, and the best place to do that is where the xmit_tpid changes,
namely in sja1105_vlan_filtering. So at the moment, exit early from the
function to avoid unnecessarily resetting the switch for each port call.
Then we'll change the xmit_tpid prior to the early exit in the next
patch.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>