rtw88: refine tx_pwr_tbl debugfs to show channel and bandwidth

Show channel and bandwidth in debugfs tx_pwr_tbl to make it easier
to understand which tx power table is shown currently, and to reduce
additional checks through other ways.

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/20211129020626.6384-1-pkshih@realtek.com