iio: adc: ad9467: add backend test mode helpers

Group the backend configurations to be done in preparing and stopping
calibration in two new helpers analogous to ad9467_testmode_set(). This
is in preparation for adding support for debugFS test_mode where
we need similar configurations as in the calibration process.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-7-4cb62852f0d0@analog.com


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