iio: adc: ad7380: do not use iio_device_claim_direct_scoped anymore

iio: adc: ad7380: do not use iio_device_claim_direct_scoped anymore

Conditionnal scoped handlers are turning out to be a real pain:
readability issues, compiler and linker handling issues among others so
rollback and remove the scoped version of iio_dvice_claim_direct_mode.

To impove code readability factorize code to set oversampling ratio.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20250108-ad7380-add-alert-support-v4-1-1751802471ba@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>