[][MAC80211][mt76][Rework tx antenna setting in testmode for litepoint]

[Description]
Change tx antenna setting in testmode for merlin (3T3R) to run on litepoint test

[Release-log]
N/A

Change-Id: I55d42003778ab21d98e1da6a0025fac08dc4ceda
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6449430
19 files changed