iptables: fix an inversion

Revisiting the original condition (viewable in git log -1 -p
v1.4.10-57-gacef604), one can notice an unforuntate inversion. This
commit corrects this.

Testcase: -A INPUT -p tcp --dport 1

Reported-by: Florian Westphal
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
1 file changed