rtlwifi: btcoex: 21a 2ant: set wifi standby when halting of entering ips

If the wifi is going to halt or entering power saving, set it standby
and allocate resource to bt.

Routine btc8821a2ant_wifi_off_hw_cfg() restored.

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>
Cc: Steven Ting <steventing@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>