rtlwifi: rtl8723be: fix ant_sel code

When ant_sel is set, we need to fill single_ant_path to select correct
antenna path.

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