bridge: fix initial packet flood if !STP

If bridge is configured with no STP and forwarding delay of 0 (which
is typical for virtualization) then when link starts it will flood all
packets for the first 20 seconds.

This bug was introduced by a combination of earlier changes:
  * forwarding database uses hold time of zero to indicate
    user wants to always flood packets
  * optimzation of the case of forwarding delay of 0 avoids the initial
    timer tick

The fix is to just skip all the topology change detection code if
kernel STP is not being used.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>