rtw88: debug: dump tx power indexes in use

Add a read entry in debugfs to dump current tx power
indexes in use for each path and each rate section.
The corresponding power bases, power by rate, and
power limit are also included.

Also this patch fixes unused function warning.

Fixes: b7414222 ("rtw88: refine flow to get tx power index")
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>