rtlwifi: btcoexist: Implement antenna selection

The previous patch added an option to rtl8723be to manually select the
antenna for those cases when only a single antenna is present, and the
on-board EEPROM is incorrectly programmed. This patch implements the
necessary changes in the Bluetooth coexistence driver.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [V4.0+]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>