tipc: Minor optimization of broadcast link transmit queue statistic

The two broadcast link statistics fields that are used to derive the
average length of that link's transmit queue are now updated only after
a successful attempt to send a broadcast message, since there is no need
to update these values when an unsuccessful send attempt leaves the
queue unchanged.

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