tipc: Minor optimization to broadcast link synchronization logic

Optimizes processing done when contact with a neighboring node is
established to avoid recording the current state of outgoing broadcast
messages if the neighboring node isn't a valid broadcast link destination,
since this state information isn't needed for such nodes.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>