greybus: control: Use gb_builtin_protocol_driver()

No need to write simple init/exit routines, use
gb_builtin_protocol_driver().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>