wifi: rtw89: debug: txpwr table access only valid page according to chip

We now support RTL8851B which has only single RF path. For chip with
single RF path, TX power page is valid only in single path section.
So, we refine debugfs txpwr table to access TX power page according
to RF path number of runtime chip. It can prevent us from reading
beyond valid sections.

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/20230531060713.57203-3-pkshih@realtek.com