net: conn_mgr: Ignore network interfaces that are not used

As network interface array size might be larger than the actual
network interface count, check this condition and ignore those
interfaces that are not in use.
We cannot know for certain how many network interfaces there
are at built time, as the total count is only available at runtime.

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