ath9k: Check for root debugfs file

Creation of the root debugfs file could have failed
for some reason, check properly before proceeding in this
case.
Signed-off-by: NSujith <Sujith.Manoharan@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/debug.c

@@ -498,6 +498,9 @@ int ath9k_init_debug(struct ath_softc *sc)
 {
 	sc->debug.debug_mask = ath9k_debug;
 
+	if (!ath9k_debugfs_root)
+		return -ENOENT;
+
 	sc->debug.debugfs_phy = debugfs_create_dir(wiphy_name(sc->hw->wiphy),
 						      ath9k_debugfs_root);
 	if (!sc->debug.debugfs_phy)