[][MAC80211][build][Fix patch fail]

[Description]
Fix patch fail
Add symbolic link of mt76-vendor for unified sigma support.
Remove unnecessary compiler flag in MT76 makefile

[Release-log]
N/A

Change-Id: Ia4e297c0648a77c4e500d926fb617cba6a1a85fb
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6140808
27 files changed