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

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

The AD2S1210 has a programmable threshold for the degradation of signal
(DOS) overrange fault. This fault is triggered when either the sine or
cosine input rises above the threshold voltage.

This patch converts the custom device DOS overrange threshold attribute
to an event rising edge threshold 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-v4-9-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>