rtw88: 8822c: add tx stbc support under HT mode

Enabling this improves tx performance for long distance transmission.
We used to enable stbc by the rx stbc cap of the associated station.
But rx cap will be masked out in ieee80211_ht_cap_ie_to_sta_ht_cap
if we do not declare tx stbc.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210713104524.47101-2-pkshih@realtek.com