rtw88: single rf path chips don't support TX STBC

Since single rf path chips don't support TX SPBC, tell mac80211
to not advertise it.

Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200603094218.19942-5-yhchuang@realtek.com