rtlwifi: btcoex: Add variable ant_div_cfg to support antenna diversity

The variable will be used by btcoex of 8822be, so we prepare this variable
in advance.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>