smsc911x: fix BUG if module is removed while interface is up

If the module is removed while its interface is up, smsc911x_stop is not
called until after smsc911x_drv_remove has disposed of phy_dev.  This
patch changes the stop method to handle this situation.

This is a different problem to the one reported by dfoley@telus.net.

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