fff-firewall: Fix match in ip6tables and add dependencies

The syntax " -m state --state " seems to be not supported anymore.

The replace should not change behavior compared to
lede-17.01-based firmware.

Added required dependency.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Fabian Bläse <fabian@blaese.de>