Some fixes for smsc usb ethernet and kernel etherusb driver (useful for Raspberry Pi):
- don't assume etherusb is #l0 (so it can coexist with another ether adapter)
- enable multicast by default for etherusb, since there's no interface available
  yet to turn it on dynamically (so ipv6 will work)
- implement promiscuous and multicast modes for smsc