greybus: legacy: use protocol pointer to determine state

Use the protocol pointer to determine the legacy connection state.

This is needed to allow core disable connections when an interface has
been hot-unplugged without the legacy protocols leaking its resources.

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>