rtlwifi: move btcoex's ant_num declaration

File halbtcoutsrc.c is a better place for routine rtl_get_hwpg_ant_num()
than file rtl_btc.c.

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>