wifi: ath9k: use IS_ERR() with debugfs_create_dir()

The debugfs_create_dir() function returns error pointers,
it never returns NULL. Most incorrect error checks were fixed,
but the one in ath9k_htc_init_debug() was forgotten.

Fix the remaining error check.

Fixes: e5facc75 ("ath9k_htc: Cleanup HTC debugfs")
Signed-off-by: Wang Ming <machel@vivo.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230713030358.12379-1-machel@vivo.com

drivers/net/wireless/ath/ath9k/htc_drv_debug.c

@@ -486,7 +486,7 @@ int ath9k_htc_init_debug(struct ath_hw *ah)
 
 	priv->debug.debugfs_phy = debugfs_create_dir(KBUILD_MODNAME,
 					     priv->hw->wiphy->debugfsdir);
-	if (!priv->debug.debugfs_phy)
+	if (IS_ERR(priv->debug.debugfs_phy))
 		return -ENOMEM;
 
 	ath9k_cmn_spectral_init_debug(&priv->spec_priv, priv->debug.debugfs_phy);