Since passing the clock is not mandatory, change the warning message to debug,
so that we avoid getting the following clock failure message on every boot:

smsc911x: Driver version 2008-10-21
smsc911x smsc911x (unregistered net_device): couldn't get clock -2
libphy: smsc911x-mdio: probed
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>