iio: adis16460: Make use of __adis_initial_startup

All of the actions done in `adis16460_initial_setup()` are now done in
`__adis_initial_startup()` so, there's no need for code duplication.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>