usb: Use well-known descriptor sizes when parsing configuration

The existing USB configuration parsing code relies on the descriptors'
own length values when reading through the configuration blob. Since the
size of those descriptors is always well-defined, we should rather use
the known sizes instead of trusting device-provided values to be
correct. Also adds some safety to potential out-of-order descriptors.

Change-Id: I16f69dfdd6793aa0fe930b5148d4521f3e5c3090
Signed-off-by: Julius Werner <jwerner@chromium.org>