iio: adc: ad7173: Clear append status bit

The previous value of the append status bit was not cleared before
setting the new value. This caused the bit to remain set after enabling
buffered mode for multiple channels and not permit further buffered
reads from a single channel after the fact.

Fixes: 76a1e6a4

 ("iio: adc: ad7173: add AD7173 driver")
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Link: https://lore.kernel.org/r/20240530-ad7173-fixes-v3-4-b85f33079e18@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>