staging: iio: ad2s1210: Drop the gpioin flag.

Drop gpioin flag which decides how the GPIOs
are controlled as the GPIOs must be outputs
for the host as per the datasheet.

Removes need for platform data so that is dropped as well.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>