iio: adc: meson-saradc: improve meson_sar_adc_read_raw_sample

After sampling there should always be only one value in the FIFO.
This also applies to averaging mode as the averaging is done
chip-internally. So we don't have to loop and let the driver
complain if there's not exactly one value in the FIFO.

If the value belongs to a different channel then don't silently
swallow the value but complain.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>