wifi: rtw89: 8922a: use right chanctx whenever possible in RFK flow

No longer access chan with hard-code RTW89_CHANCTX_X whenever possible.
Instead, obtain the right chanctx from somewhere and use it in RTL8922A
RFK (RF calibration) related code.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240819091724.33730-7-pkshih@realtek.com