greybus: legacy: add private driver data

Allocate private data structure at probe and release at disconnect.

The private data will be used to track the connections managed by the
driver, but for now only holds a count of the number of cports the
bundle has.

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>