mwan3: fix rtmon routing table function generation

If the uci option family is not set in the interface section, then there
is no default value set as in the `config_load / config_get` API.
The problem here is that if the family is not set, the default value ipv4
is normaly assumed. But the comparison fails here because the value is empty
and therefore the dedicated routing table for this interface is not compared
with the other routes from the main table and so not updated.

To fix this set the default value for this config option which is`false`
for enabled and `ipv4` for family.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>