rtw88: correct power limit selection

If phy rate is decreased, sub bandwidth may be chosen by RA.
We consider possible power limits and apply the min one;
otherwise, the tx power index may be larger than spec.

And we cross-reference power limits of vht and ht with
20/40M bandwidth in 5G to avoid values are not assigned.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>