greybus: hd: make svc-connection life time coincide with host-device

Create the svc-connection as part of the host device, and destroy it in
the host-device destructor.

The svc-connection is enabled when registering the host device, and
disabled when the host device is deregistered.

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>