iio: adc: ad7380: do not store osr in private data structure

Since regmap cache is now enabled, we don't need to store the
oversampling ratio in the private data structure.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20250108-ad7380-add-alert-support-v4-3-1751802471ba@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>