smsc911x: don't clobber driver_data

smsc911x uses driver_data to store our net_device, don't overwrite this
with the mii_bus.

Reported and fixed by dfoley@telus.net.

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