rtlwifi: btcoex: Fix some static warnings from Sparse

Add 'static' or declaration to resolve the warnings, and remove two unused
functions halbtc_set_macreg() and halbtc_get_macreg() exposed when they
were made static.

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