net: conn: Check connection address family properly

When a connection handler was registered, the checker function
introduced in commit 43b37cef ("Check duplicate UDP/TCP connection
handlers") did not check the address family (IPv4 or IPv6) of
the local end point properly. This caused duplicate connection
error to be returned to the caller.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>