iio: adc: ad7173: Fix sampling frequency setting

This patch fixes two issues regarding the sampling frequency setting:
-The attribute was set as per device, not per channel. As such, when
 setting the sampling frequency, the configuration was always done for
 the slot 0, and the correct configuration was applied on the next
 channel configuration call by the LRU mechanism.
-The LRU implementation does not take into account external settings of
 the slot registers. When setting the sampling frequency directly to a
 slot register in write_raw(), there is no guarantee that other channels
 were not also using that slot and now incorrectly retain their config
 as live.

Set the sampling frequency attribute as separate in the channel templates.
Do not set the sampling directly to the slot register in write_raw(),
just mark the config as not live and let the LRU mechanism handle it.
As the reg variable is no longer used, remove it.

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-v3-5-b85f33079e18@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>