greybus: Add flags to struct gb_protocol

This helps in removing special per-protocol code, with the help of
generic flags passed by protocol drivers.

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