This patch updates the USB audio class driver to use the usb_ifnum_to_if()
and usb_altnum_to_altsetting() routines, thereby removing assumptions
about which interface or altsetting is stored in which array entry.

It also simplifies the driver's probe() routine by using the raw
configuration descriptor already loaded into memory instead of reading the
descriptor from the device.  Now, either the current driver has a bug and
never deallocates the buffer used to hold the descriptor, or else I've
introduced a double-free error.  There's no obvious place where the buffer
gets freed, but it's hard to be certain.

It would be good if someone could try out this patch.  I can't test it,
not having any USB audio devices handy.  If the double-free error is
present, it will show up when the device is disconnected and the
configuration data is released.