iio: adc: ad7606: fix/persist oversampling_ratio setting

When the mutexes were reworked to guards, the caching of the
oversampling_ratio values was removed by accident.

The main effect of this change is that, after setting the
oversampling_ratio value, reading it back would result in the initial value
(of 1).
The value would get sent to the device correctly though.

Fixes 2956979d: ("iio: adc: ad7606: switch mutexes to guard")

Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241025095939.271811-2-aardelean@baylibre.com


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