rtlwifi: Add modifier static to functions reported by sparse

sparse reports some functions were not declared, so add 'static' as
modifier. Remove an unused function btc8821a1ant_is_wifi_status_changed()
exposed due to '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>