iio: adc: at91-sama5d2: Replace 0 with NULL when clearing some pointers.

Cleans up the sparse warning:

CHECK   drivers/iio/adc/at91-sama5d2_adc.c
drivers/iio/adc/at91-sama5d2_adc.c:1486:31: warning: Using plain integer as NULL pointer
drivers/iio/adc/at91-sama5d2_adc.c:1509:31: warning: Using plain integer as NULL pointer

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>