staging: iio: ad2s1210: Remove platform data NULL check in probe

The driver only needs to access GPIOs via GPIOLIB. And SPI can be
configured via device-tree or ACPI.

The platform data is a remnant from another time, and should not cause the
driver to error out during probing if it is NULL.
Also, it's not used.

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