greybus: greybus_protocols: Pack structure as required by the module firmware

These structures are expected to be packed by the module firmware code,
but the kernel wasn't following it until now.

Its all working currently because compiler doesn't add any pad bytes for
these structures, as their elements are already aligned to their size.

But these structures can change in future and we better mark them
packed.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>