iio: st_sensors: remove reference to parent device object on st_sensor_data

iio: st_sensors: remove reference to parent device object on st_sensor_data

The idea behind it, is that all devm_ calls in ST sensors are bound to the
parent device object.

However, the reference to that object is kept on both the st_sensor_data
struct and the IIO object parent (indio_dev->dev.parent).

This change only adds a bit consistency and uses the reference stored on
indio_dev->dev.parent, to enforce the assumption that all ST sensors' devm_
calls are bound to the same reference as the one store on st_sensor_data.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210823112204.243255-6-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>