smsc9420: prevent BUG() if ethtool is called with interface down

This patch fixes a null pointer dereference BUG() if ethtool is used on
an smsc9420 interface while it is down, because the phy_dev is only
allocated while the interface is up.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>