counter: Provide a wrapper to access device private data

counter: Provide a wrapper to access device private data

For now this just wraps accessing struct counter_device::priv. However
this is about to change and converting drivers to this helper
individually makes fixing device lifetime issues result in easier to
review patches.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211230150300.72196-5-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>