rtw88: 8723d: Set IG register for CCK rate

DIG sets only one IG register for most chips, but 8723D need to set
additional register for CCK rate.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200504105010.10780-8-yhchuang@realtek.com