bridge: fix hello and hold timers starting/stopping

bridge: fix hello and hold timers starting/stopping

Current bridge code incorrectly handles starting/stopping of hello and
hold timers during STP enable/disable.

1. Timers are stopped in br_stp_start() during NO_STP->USER_STP
   transition. The timers are already stopped in NO_STP state so
   this is confusing no-op.

2. During USER_STP->NO_STP transition the timers are started. This
   does not make sense and is confusion because the timer should not be
   active in NO_STP state.

Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Ivan Vecera <[email protected]>
Reviewed-by: Xin Long <[email protected]>
Acked-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>