iio: adc: ad4695: simplify getting oversampling_ratio

iio: adc: ad4695: simplify getting oversampling_ratio

We already have a local variable that holds a pointer to
st->channels_cfg[chan->scan_index]. Use that to simplify the code.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250218-iio-adc-ad4695-fix-out-of-bounds-array-access-v1-2-57fef8c7a3fd@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>