staging: iio: ad5933: use dev_get_platdata()

Use dev_get_platdata() for retrieving the platform data instead of
accessing dev->platform_data directly.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>