rtlwifi: btcoex: Add power_on_setting routine

After mac power-on sequence, wifi will start to work so notify btcoex the
event to configure registers especially related to antenna. This will not
only help to assign antenna but also to yield better user experience.

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