iio: adis16480: Make use of __adis_initial_startup

All actions done in `adis16480_initial_setup()` are now done in
`__adis_initial_startup()` so, there's no need for code duplication.
Furthermore, the call to `adis16480_initial_setup()` is done before any
device configuration since the device will be reset if not already (via
rst pin). This is actually fixing a potential bug since `adis_reset()` was
being called after configuring the device which is obviously a problem.

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>