staging: iio: resolver: ad2s1210: convert DOS mismatch threshold to event attr

staging: iio: resolver: ad2s1210: convert DOS mismatch threshold to event attr

The AD2S1210 has a programmable threshold for the degradation of signal
(DOS) mismatch fault. This fault is triggered when the difference in
voltage between the sine and cosine inputs exceeds the threshold. In
other words, when the magnitude of sine and cosine inputs are equal,
the AC component of the monitor signal is zero and when the magnitudes
of the sine and cosine inputs are not equal, the AC component of the
monitor signal is the difference between the sine and cosine inputs.
So the fault occurs when the magnitude of the AC component of the
monitor signal exceeds the DOS mismatch threshold voltage.

This patch converts the custom device DOS mismatch threshold attribute
to an event magnitude attribute on the monitor signal channel.

The attribute now uses millivolts instead of the raw register value in
accordance with the IIO ABI.

Emitting the event will be implemented in a later patch.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-10-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>