mac80211: fix sparse integer as NULL pointer warning

drivers/net/wireless/mac80211_hwsim.c:503:20: warning: Using plain integer as NULL pointer
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -500,7 +500,7 @@ failed_hw:
 	device_unregister(data->dev);
 failed_drvdata:
 	ieee80211_free_hw(hw);
-	hwsim_radios[i] = 0;
+	hwsim_radios[i] = NULL;
 failed:
 	mac80211_hwsim_free();
 	return err;