iio: adc: ad7606_spi: add offload scan mask check

iio: adc: ad7606_spi: add offload scan mask check

Validate the scan mask when SPI offloading is being used.

Since this family of ADCs is simultaneous sampling, there isn't a way
to selectively disable channels when reading sample data. (Technically,
AD7616 has a sequencer so the driver could have some control, but that
is for another day).

For "regular" IIO triggered buffer reads, this isn't a problem and the
IIO core will demux the data and ignore data from disabled channels.
However, since SPI offloading is done completely in hardware, we don't
have a way to do the same. So before this patch, if less than all
channels were enabled, the data would be misplaced in the buffer.

By adding a check in update_scan_mode, we can fail to enable the buffer
instead of having bad data returned to userspace.

Fixes: e96d35faf357 ("iio: adc: ad7606: add SPI offload support")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250502-iio-adc-ad7606_spi-fix-offload-scan-mask-check-v2-1-e70c6d71baa3@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>