This makes the new CDC Ethernet code handle more devices:

  - Uses the active config, not just the default one, if it's
    coping "descriptors in wrong place" quirk.  (bugfix)

  - Uses usb_ifnum_to_if() to get interfaces.  (bugfix)

  - AMBIT USB cable modems have bogus CDC Union descriptors;
    workaround by switching master and slave.  (add quirk)

  - To make it easier the next time we run into firmware
    that violates the class spec, add debug messages saying
    exactly why it's giving up on a given CDC device.

Net result, this code now handles at least one more
cable modem design.