[][MAC80211][mt76][use random payload for testmode tx]

[Description]
Change testmode tx payload from fixed to random, which aligns EVM with
proprietary driver. The patch will be upstreamed after doing more tests.

[Release-log]
N/A

Change-Id: I2ff1cafa3f108beb628a47a3ebb81182514e1009
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6216190
1 file changed