staging: iio: resolver: ad2s1210: convert LOS threshold to event attr

staging: iio: resolver: ad2s1210: convert LOS threshold to event attr

The AD2S1210 has a programmable threshold for the loss of signal (LOS)
fault. This fault is triggered when either the sine or cosine input
falls below the threshold voltage.

This patch converts the custom device LOS threshold attribute to an
event falling edge threshold attribute on a new monitor signal channel.
The monitor signal is an internal signal that combines the amplitudes
of the sine and cosine inputs as well as the current angle and position
output. This signal is used to detect faults in the input signals.

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-v4-8-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>