greybus: manifest: use bundle's embedded interface pointer

An initialized bundle structure contains a pointer to its
interface.  Because of this there's no need to provide
the interface pointer to gb_manifest_parse_cports().  This
also precludes the possibility of passing a bad interface
pointer.

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