cfg80211: add wiphy_idx_valid to check for wiphy_idx sanity

This will later be used by others, for now make use of it in
cfg80211_drv_by_wiphy_idx() to return early if an invalid
wiphy_idx has been provided.
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/core.c

@@ -42,6 +42,9 @@ cfg80211_drv_by_wiphy_idx(int wiphy_idx)
 {
 	struct cfg80211_registered_device *result = NULL, *drv;
 
+	if (!wiphy_idx_valid(wiphy_idx))
+		return NULL;
+
 	list_for_each_entry(drv, &cfg80211_drv_list, list) {
 		if (drv->wiphy_idx == wiphy_idx) {
 			result = drv;
@@ -225,7 +228,7 @@ struct wiphy *wiphy_new(struct cfg80211_ops *ops, int sizeof_priv)
 
 	drv->wiphy_idx = wiphy_counter++;
 
-	if (unlikely(drv->wiphy_idx < 0)) {
+	if (unlikely(!wiphy_idx_valid(drv->wiphy_idx))) {
 		wiphy_counter--;
 		mutex_unlock(&cfg80211_drv_mutex);
 		/* ugh, wrapped! */

net/wireless/core.h

@@ -63,6 +63,13 @@ struct cfg80211_registered_device *wiphy_to_dev(struct wiphy *wiphy)
 	return container_of(wiphy, struct cfg80211_registered_device, wiphy);
 }
 
+/* Note 0 is valid, hence phy0 */
+static inline
+bool wiphy_idx_valid(int wiphy_idx)
+{
+	return (wiphy_idx >= 0);
+}
+
 extern struct mutex cfg80211_drv_mutex;
 extern struct list_head cfg80211_drv_list;