staging:iio:ad2s90: Remove spi setup that should be done via dt

The ad2s90 driver currently sets some spi settings (max_speed_hz and
mode) at ad2s90_probe. Since the maximum frequency is a required element
in DT binding for spi slave devices and because the spi mode for the
device can be either (0,0) or (1,1), these settings should be handled
via device tree, not in the driver's code. This patch removes them from
the probe function.

Note: The way in which the mentioned spi settings need to be specified
on the ad2s90's node of a device tree will be documented in the future
patch "dt-bindings:iio:resolver: Add docs for ad2s90".

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>