greybus: fix manifest parsing problem with descriptor payload

The internal struct manifest_desc needs the data payload, rather
than the entire descriptor with header to be populated into the
data field. Also fix two places where the parser was trying to
extract the entire descriptor with header for the data payload
field.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>