staging:iio:ad2s90: Move device registration to the end of probe

Previously, devm_iio_device_register was being called before the
spi_setup call and the spi_device's max_speed_hz and mode assignments.
This could lead to a race condition since the driver was still being
set up after it was already made ready to use. To fix it, this patch
moves the device registration to the end of ad2s90_probe.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>