greybus: connection: refactor connection enable and disable

Refactor connection enable and disable.

This is a step towards removing the legacy-protocol handling from core.

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