rtlwifi: btcoex: Add switch band notify for btc

BT shares 2.4G band but not 5G band, so inform current band to btcoex to
setup antenna properly.

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>