smsc95xx: use usbnet->driver_priv

Using `void *driver_priv` instead of `unsigned long data[]` is more
straightforward way to recover the `struct smsc95xx_priv *` from the
`struct net_device *`.

Signed-off-by: Andre Edich <andre.edich@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>