greybus: manifest: descriptor size should be >= header size

We are calculating descriptors expected size differently based on the type of
descriptor, that's fine but at few places we aren't taking size of the header
into account. And that looks wrong.

Lets make sure it is atleast as big as descriptor's header.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>