iio: adc: at91: Use dev_get_drvdata()

Using dev_get_drvdata directly.

Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>