rtw88: 8723d: Add DIG parameter

To improve user experience in field, we need DIG to adjust RX initial gain
depends on field situation. Since each chip has its own register address,
this commit defines 8723d specific address.

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/20200504105010.10780-2-yhchuang@realtek.com