greybus: svc: fix ida memory leak

The device-id map was never deallocated on SVC-connection tear down.

Also make the map per-SVC-connection (there should still be only one)
rather than use a global pointer.

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>