rtw89: packed IGI configuration flow into function for DIG feature

Refinement of DIG flow, a mechanism to adjust Rx gain for better Rx
performance, by packing IGI(initial gain index) configuration flow into
one function.

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/20220318023214.32411-6-pkshih@realtek.com