greybus: operation: clean up request handler

Clean up the incoming request handler somewhat by passing a properly
aligned header and dropping the now redundant id and type parameters.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>