ieee802154: fakelb: switch from BUG_ON() to WARN_ON() on problem

The check is valid but it does not warrant to crash the kernel. A
WARN_ON() is good enough here.
Found by checkpatch.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>

drivers/net/ieee802154/fakelb.c

@@ -49,7 +49,7 @@ struct fakelb_phy {
 
 static int fakelb_hw_ed(struct ieee802154_hw *hw, u8 *level)
 {
-	BUG_ON(!level);
+	WARN_ON(!level);
 	*level = 0xbe;
 
 	return 0;