rocker: put port in FORWADING state after leaving bridge

Cleanup the port forwarding state transitions for the cases when the port
joins or leaves a bridge, or is brought admin UP or DOWN.  When port is
bridged, we can rely on bridge driver putting port in correct state using
STP callback into port driver, regardless if bridge is enabled for STP or not.
When port is not bridged, we can reuse some of the STP code to enabled or
disable forwarding depending on UP or DOWN.

Tested by trying all the transitions from bridge/not bridge, and UP/DOWN, and
verifying port is in the correct forwarding state after each transition.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>