greybus: bundle: Create bundles using bundle descriptors

Currently we are creating bundles based on interface descriptors. An interface
can have one or more bundles associated with it and so a bundle must be created
based on a bundle descriptor.

Also get class_type from bundle descriptor.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>