net/aquantia-atlantic-kmod: Fix build on CURRENT

This patch enables the port to build a working if_atlantic.ko on
13-CURRENT, broken since r353868 (Git commit 19e09f447fd9), where
the if_multi_apply and if_multiaddr_count KPIs were removed.

PR: 252642
Submitted by: Michael Chiu <nyan-at-myuji.xyz>
Approved by: koobs (maintainer)
MFH: 2021Q1 (blanket: build fix)
Differential_Revision: https://reviews.freebsd.org/D28135