rtw89: refine DIG feature to support 160M and CCK PD

DIG, which is short for dynamic initial gain, is used to adjust gain to get
good RX performance. CCK PD feature, a mechanism that adjusts 802.11b CCK
packet detection(PD) power threshold based on environment noisy level in
order to avoid false alarm. Also, refine related variable naming.

Signed-off-by: Johnson Lin <johnson.lin@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/20220121075555.12457-1-pkshih@realtek.com