tipc: Eliminate useless return value when disabling a bearer

Modifies bearer_disable() to return void since it always indicates
success anyway.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>