tipc: Fix bug in connection setup via native API

This patch fixes a bug that prevented TIPC from receiving a
connection setup request message on a native TIPC port.
The revised connection setup logic ensures that validation
of the source of a connection-based message is skipped if
the port is not yet connected to a peer.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>