staging: iio: resolver: ad2s1210: convert LOT threshold attrs to event attrs

staging: iio: resolver: ad2s1210: convert LOT threshold attrs to event attrs

The AD2S1210 monitors the internal error signal (difference between
estimated angle and measured angle) to determine a loss of position
tracking (LOT) condition. When the error value exceeds a threshold, a
fault is triggered. This threshold is user-configurable.

This patch converts the custom lot_high_thrd and lot_low_thrd attributes
in the ad2s1210 driver to standard event attributes. This will allow
tooling to be able to expose these in a generic way.

Since the low threshold determines the hysteresis, it requires some
special handling to expose the difference between the high and low
register values as the hysteresis instead of exposing the low register
value directly.

The attributes also return the values in radians now as required by the
ABI.

Actually emitting the fault event will be done in a later patch.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-7-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>