rc.d/ipfilter: ipfilter must be enabled for options to take

ipfilter options are erased and reset to default when ipfilter is
disabled. This results in nullifying options from rc.conf that were
previously set.

8d6feaaaa26f, which added this code, was incorrect as it was for a bug in
ipfilter 4.2.28 and no longer applies to ipfilter 5.1.2.

Fixes: 8d6feaaaa26f

(cherry picked from commit cc1e4aae5a67a20f3c0fff13612364e6e4404f93)