greybus: greybus_protocols: Pack all request/response structure

These structures are exchanged between the AP and the module and must be
packed to avoid any unwanted holes.

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>