rtl8xxxu: fix warnings for symbol not declared

Fix the following sparse warnings.
sparse: symbol 'rtl8723bu_set_coex_with_type' was not declared.
Should it be static?
sparse: symbol 'rtl8723bu_update_bt_link_info' was not declared.
Should it be static?
sparse: symbol 'rtl8723bu_handle_bt_inquiry' was not declared.
Should it be static?
sparse: symbol 'rtl8723bu_handle_bt_info' was not declared.
Should it be static?

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>