rtw88: refine flow to get tx power index

Add a structure for power parameters including base,
offset, limit and a function to get tx power parameters.
Then, refine flow to get tx power index through the
function.

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>