iio: adc: ad7606: move the software mode configuration

iio: adc: ad7606: move the software mode configuration

This is a preparation for the intoduction of the sofware functions in
the iio backend version of the driver.
The software mode configuration must be executed once the channels are
configured, and the number of channels is known. This is not the case
before iio-backend's configuration is called, and iio backend version of
the driver does not have a timestamp channel.
Also the sw_mode_config callback is configured during the iio-backend
configuration.
For clarity purpose, I moved the entire block instead of just the
concerned function calls.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-2-160df18b1da7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>