greybus: connection: unbind protocol at exit

Unbind protocol at connection exit rather than when the connection is
destroyed.

Now a protocol is only bound while a connection is enabled.

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>