mwifiex: fix NULL pointer dereference error

In mwifiex_enable_hs, we need to check if
priv->wdev.wiphy->wowlan_config is NULL before accessing its member.
This sometimes cause kernel panic when suspend/resume.
Signed-off-by: Wei-Ning Huang <wnhuang@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/mwifiex/sta_ioctl.c

@@ -509,7 +509,8 @@ int mwifiex_enable_hs(struct mwifiex_adapter *adapter)
 
 	if (priv && priv->sched_scanning) {
 #ifdef CONFIG_PM
-		if (!priv->wdev.wiphy->wowlan_config->nd_config) {
+		if (priv->wdev.wiphy->wowlan_config &&
+		    !priv->wdev.wiphy->wowlan_config->nd_config) {
 #endif
 			mwifiex_dbg(adapter, CMD, "aborting bgscan!\n");
 			mwifiex_stop_bg_scan(priv);