This patch changes the usb_parse_interface() routine so that it only
handles a single interface/altsetting descriptor at a time, rather than
trying to handle all the altsettings for an interface at once.  Besides
shrinking the code slightly, this has the advantage of not requiring the
interfaces to be listed in order or all the altsetting descriptors for an
interface to be contiguous.  While there probably aren't any devices that
have _discontiguous_ altsetting descriptors, there's no harm in allowing
it -- particularly since doing so provides an overall simplification.

This is another of those hard-to-read patches.  It moves most of the body
of the usb_parse_interface() function out of a loop, thereby changing the
indentation level without actually altering the code.