greybus: manifest: rework cport parsing

Rework the the code that parses the manifest for CPorts associated
with a bundle so it only touches each manifest descriptor once.
(Previously the list was scanned from the beginning repeatedly
until all bundle CPorts were found.)  Shorten the name of the
descriptor variable, to avoid line wrap.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>