smsc911x: Change clock warning message to debug level

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>