rtw88: 8723d: fix sparse warnings for power tracking

sparse warnings: (new ones prefixed by >>)

>> drivers/net/wireless/realtek/rtw88/rtw8723d.c:1899:6: sparse: sparse:
symbol 'rtw8723d_pwr_track' was not declared. Should it be static?
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200512103534.5889-1-yhchuang@realtek.com

drivers/net/wireless/realtek/rtw88/rtw8723d.c

@@ -1896,7 +1896,7 @@ static void rtw8723d_phy_pwrtrack(struct rtw_dev *rtwdev)
 		rtw8723d_phy_calibration(rtwdev);
 }
 
-void rtw8723d_pwr_track(struct rtw_dev *rtwdev)
+static void rtw8723d_pwr_track(struct rtw_dev *rtwdev)
 {
 	struct rtw_efuse *efuse = &rtwdev->efuse;
 	struct rtw_dm_info *dm_info = &rtwdev->dm_info;