ath10k: fix debugfs_create_dir() checking

The function may return an -ENODEV if debugfs is
disabled in kernel. This should originally be
guarded by ath10k's Kconfig but it still makes
sense to check for the non-NULL errno return
value.
Reported-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath10k/debug.c

@@ -1176,8 +1176,12 @@ int ath10k_debug_register(struct ath10k *ar)
 {
 	ar->debug.debugfs_phy = debugfs_create_dir("ath10k",
 						   ar->hw->wiphy->debugfsdir);
-	if (!ar->debug.debugfs_phy)
+	if (IS_ERR_OR_NULL(ar->debug.debugfs_phy)) {
+		if (IS_ERR(ar->debug.debugfs_phy))
+			return PTR_ERR(ar->debug.debugfs_phy);
+		else
 			return -ENOMEM;
+	}
 
 	INIT_DELAYED_WORK(&ar->debug.htt_stats_dwork,
 			  ath10k_debug_htt_stats_dwork);