wifi: rtw89: TX power stuffs replace confusing naming of _max with _num

Some old declarations about TX power stuffs were named with confusing
`_max`. But, they mean "the number of". So we change them to be named
with `_num`.

(No logic is changed.)

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/20230616060523.28396-1-pkshih@realtek.com