greybus: interface: remove unused drvdata helpers

Remove the unused interface drvdata helpers along with some dubious
comments about public and private definitions.

Greybus drivers bind to bundles and should be using the
greybus_set_drvdata and greybus_get_drvdata helpers.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>