tipc: change tipc_net_start routine return value type

Since now tipc_net_start() always returns a success code - 0, its
return value type should be changed from integer to void, which can
avoid unnecessary check for its return value.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>