firewall3: fix typo that affects ICMPv6 rules with numeric icmp_type
Problem can be reproduced with a rule like this:
option src 'wan'
option family 'ipv6'
option proto 'icmp'
option icmp_type '128'
option target 'DROP'
The resulted rule will set --icmpv6-type to 128/255.
Signed-off-by: Alin Nastac <alin.nastac@gmail.com>