rtlwifi: btcoexist: Fix breakage of ant_sel for rtl8723be

In commit bcd37f4a ("rtlwifi: btcoex: 23b 2ant: let bt transmit when
hw initialisation done"), there is an additional error when the module
parameter ant_sel is used to select the auxilary antenna. The error is
that the antenna selection is not checked when writing the antenna
selection register.

Fixes: bcd37f4a ("rtlwifi: btcoex: 23b 2ant: let bt transmit when hw initialisation done")
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ping-Ke Shih <pkshih@realtek.com>
Cc: Yan-Hsuan Chuang <yhchuang@realtek.com>
Cc: Birming Chiu <birming@realtek.com>
Cc: Shaofu <shaofu@realtek.com>
Cc: Steven Ting <steventing@realtek.com>
Cc: Stable <stable@vger.kernel.org> # 4.12+
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>