net: caif: replace BUG_ON with recovery code

In caif_xmit, there is a crash if the ptr dev is NULL. However, by
returning the error to the callers, the error can be handled. The
patch fixes this issue.
Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/caif/caif_serial.c

@@ -270,7 +270,9 @@ static int caif_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct ser_device *ser;
 
-	BUG_ON(dev == NULL);
+	if (WARN_ON(!dev))
+		return -EINVAL;
+
 	ser = netdev_priv(dev);
 
 	/* Send flow off once, on high water mark */