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>
diff --git a/net/wireless/core.c b/net/wireless/core.c
index 2b3e786..35d457b 100644
--- a/net/wireless/core.c
+++ b/net/wireless/core.c
@@ -42,6 +42,9 @@
 {
 	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 @@
 
 	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! */
diff --git a/net/wireless/core.h b/net/wireless/core.h
index 1783781..4f2e0fe 100644
--- a/net/wireless/core.h
+++ b/net/wireless/core.h
@@ -63,6 +63,13 @@
 	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;