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