iio: adc: meson-saradc: fix potential crash in meson_sar_adc_clear_fifo

meson_sar_adc_clear_fifo passes a 0 as value-pointer to regmap_read().
In case of the meson-saradc driver this ends up in regmap_mmio_read(),
where the value-pointer is de-referenced unconditionally to assign the
value which was read.
Fix this by passing an actual pointer, even though all we want to do is
to discard the value.

As a side-effect this fixes a sparse warning ("Using plain integer as
NULL pointer") as reported by Paolo Cretaro.

Fixes: 3adbf342 ("iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs")
Reported-by: Paolo Cretaro <paolocretaro@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>