netfilter: ipset: Check IPSET_ATTR_PORT only once

We do not need to check tb[IPSET_ATTR_PORT] != NULL before
retrieving port, as this attribute is known to exist due to
ip_set_attr_netorder() returning true only when attribute
exists and it is in network byte order.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>