cfg80211: fix default HE tx bitrate mask in 2G band

In 2G band, a HE sta can only supports HT and HE, but not supports VHT.
In this case, default HE tx bitrate mask isn't filled, when we use iw to
set bitrates without any parameter.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://lore.kernel.org/r/20210609075944.51130-1-pkshih@realtek.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>