iio: adc: ad7173: prevent scan if too many setups requested

iio: adc: ad7173: prevent scan if too many setups requested

Add a check to ad7173_update_scan_mode() to ensure that we didn't exceed
the maximum number of unique channel configurations.

In the AD7173 family of chips, there are some chips that have 16
CHANNELx registers but only 8 setups (combination of CONFIGx, FILTERx,
GAINx and OFFSETx registers). Since commit 92c247216918 ("iio: adc:
ad7173: fix num_slots"), it is possible to have more than 8 channels
enabled in a scan at the same time, so it is possible to get a bad
configuration when more than 8 channels are using unique configurations.
This happens because the algorithm to allocate the setup slots only
takes into account which slot has been least recently used and doesn't
know about the maximum number of slots available.

Since the algorithm to allocate the setup slots is quite complex, it is
simpler to check after the fact if the current state is valid or not.
So this patch adds a check in ad7173_update_scan_mode() after setting up
all of the configurations to make sure that the actual setup still
matches the requested setup for each enabled channel. If not, we prevent
the scan from being enabled and return an error.

The setup comparison in ad7173_setup_equal() is refactored to a separate
function since we need to call it in two places now.

Fixes: 92c247216918 ("iio: adc: ad7173: fix num_slots")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250722-iio-adc-ad7173-fix-setup-use-limits-v2-1-8e96bdb72a9c@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>