extensions: devgroup: fix showing and saving of dst-group

Closes bugzilla: https://bugzilla.netfilter.org/show_bug.cgi?id=985

The --dst-group parameter in devgroup extensions lists and saves
incorrectly its value. --dst-group always shows "0x0/0x0".

This is an example:

 # iptables -I FORWARD -m devgroup --dst-group 200 -j ACCEPT

 # iptables -L FORWARD
 Chain FORWARD (policy ACCEPT)
 target     prot opt source               destination
 ACCEPT     all  --  anywhere             anywhere     src-group 0x64 dst-group 0x0/0x0

 # iptables -S FORWARD
 -P FORWARD ACCEPT
 -A FORWARD -m devgroup --dst-group 0x0/0x0 -j ACCEPT

Reporte-by: Axinchan <axinchan@cnrouter.com>
Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>