greybus: firmware: drop 'stage' from ready-to-boot request

The spec says that it doesn't need it, so dropping it.

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