iio: adc: meson-saradc: use NULL instead of 0 for pointer

Fix sparse warning: Using plain integer as NULL pointer

Signed-off-by: Paolo Cretaro <paolocretaro@gmail.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>