greybus: connection: add control connection flag

Add control connection flag which will be set for control connections.

Also add a helper function to test if the flag is set.

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