rtw88: recover rates of rate adaptive mechanism

Some APs like CMW270 only support one phyrate and the function
rtw_update_rate_mask could disable that rate. To fix such problem, we
restore the rate mask if we find that the rate_mask is empty.

Signed-off-by: Chien-Hsun Liao <ben.liao@realtek.com>
Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220208082427.42433-2-pkshih@realtek.com