greybus: control: make disconnecting a core operation

Make the control-protocol disconnecting operation a "core" operation.

This is needed to be able to use the operation in the new connection
tear-down sequence of control connections, which moves disconnecting
after the flush barrier.

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