rtlwifi: btcoex: 21a 2ant: macro for bt rssi threshold

Using macro to control the bt threshold.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Pkshih <pkshih@realtek.com>
Cc: Birming Chiu <birming@realtek.com>
Cc: Shaofu <shaofu@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>