ng_ipfw(4): add missing change after previous commit

The function ng_ipfw_input() used to enjoy implicit
32->16 bits truncation of its second argument.
Make it explicit to recover from the breakage.

PR: 281082
Reported by: Ruben van Staveren <ruben@verweg.com>
Tested by: Ruben van Staveren <ruben@verweg.com>
MFC after: 3 days
Fixes: 20e1f207cc789a28783344614d6d1d1c639c5797