wifi: rtw89: txpwr: concentrate channel related control to top

For future support on multiple channels, it would be disturbing if we
still allow scattered leaf functions of TX power to query and manage
channel related control by themselves.

So, query rtw89_chan only on top functions. Then, pass it via functions
to make sure that the values coming from the same struct rtw89_chan.

Besides, fix rtw8852a_set_txpwr_offset() from rtw8852a_set_txpwr_ctrl()
to rtw8852a_set_txpwr(). TX power offset should consider current band,
so move it to chip_ops::set_txpwr() which will be called every time that
channel is set.

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/20220809104952.61355-6-pkshih@realtek.com