rtw88: don't treat NULL pointer as an array

I'm not a standards expert, but this really looks to be undefined
behavior, when chip->dig_cck may be NULL. (And, we're trying to do a
NULL check a few lines down, because some chip variants will use NULL.)

Fixes: fc637a86 ("rtw88: 8723d: Set IG register for CCK rate")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821211716.1631556-1-briannorris@chromium.org

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

@@ -147,12 +147,13 @@ void rtw_phy_dig_write(struct rtw_dev *rtwdev, u8 igi)
 {
 	struct rtw_chip_info *chip = rtwdev->chip;
 	struct rtw_hal *hal = &rtwdev->hal;
-	const struct rtw_hw_reg *dig_cck = &chip->dig_cck[0];
 	u32 addr, mask;
 	u8 path;
 
-	if (dig_cck)
+	if (chip->dig_cck) {
+		const struct rtw_hw_reg *dig_cck = &chip->dig_cck[0];
 		rtw_write32_mask(rtwdev, dig_cck->addr, dig_cck->mask, igi >> 1);
+	}
 
 	for (path = 0; path < hal->rf_path_num; path++) {
 		addr = chip->dig[path].addr;