rtw88: fix sparse warnings for download firmware routine

sparse warnings: (new ones prefixed by >>)

>> drivers/net/wireless/realtek/rtw88/mac.c:653:5: sparse: sparse:
symbol '__rtw_download_firmware' was not declared. Should it be static?
>> drivers/net/wireless/realtek/rtw88/mac.c:817:5: sparse: sparse:
symbol '__rtw_download_firmware_legacy' was not declared. Should it be
static?

Fixes: 15d2fcc6 ("rtw88: add legacy firmware download for 8723D devices")
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/20200424101255.28239-1-yhchuang@realtek.com