rtlwifi: btcoex: extend get_wifi_bw to support bandwidth 80M

The rtlwifi newer ICs support 80M bandwidth in 5G band, so extend
get_wifi_bw() to know bandwidth 80M that helps btcoex to make correct
decisions.

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>