[][MAC80211][app][Change netfilter.mk for backport flowoffload]

[Description]
Change netfilter.mk for backport flowoffload.
Build conflict with nf_flow_table.ko.
Remove it if enable flowoffload

[Release-log]
N/A

Change-Id: Id49d2d23d0363259cafa225aa739372a67ecc078
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6222693
3 files changed