wifi: rtw89: mac: get TX power control register according to chip gen

There are two difference between Wi-Fi 6 and Wi-Fi 7 chips.
1. Address range of TX power control register
2. Checking code to get a TX power control register

So, separate the implementation of them, access according to
chip generation, and rename original things with a suffix `_ax`.

Signed-off-by: Zong-Zhe Yang <kevin_yang@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/20231003015446.14658-2-pkshih@realtek.com