mwan3: fix IPv6 routing add handling

This fixes routing handling. Introduced with the last version update.
The following message disappears on the shell
when mwan3 is called with 'mwna3 restart`.

`Error: Invalid gateway address.`

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
[aaronjg@stanford.edu: fully unset variable and handle ipv4 as well]
Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>