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: 15d2fcc6b2 ("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

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

@@ -696,7 +696,8 @@ static void download_firmware_end_flow(struct rtw_dev *rtwdev)
 	rtw_write16(rtwdev, REG_MCUFW_CTRL, fw_ctrl);
 }
 
-int __rtw_download_firmware(struct rtw_dev *rtwdev, struct rtw_fw_state *fw)
+static int __rtw_download_firmware(struct rtw_dev *rtwdev,
+				   struct rtw_fw_state *fw)
 {
 	struct rtw_backup_info bckp[DLFW_RESTORE_REG_NUM];
 	const u8 *data = fw->firmware->data;
@@ -860,7 +861,8 @@ static int download_firmware_validate_legacy(struct rtw_dev *rtwdev)
 	return -EINVAL;
 }
 
-int __rtw_download_firmware_legacy(struct rtw_dev *rtwdev, struct rtw_fw_state *fw)
+static int __rtw_download_firmware_legacy(struct rtw_dev *rtwdev,
+					  struct rtw_fw_state *fw)
 {
 	int ret = 0;