iio: core: add function to retrieve active_scan_mask index

iio: core: add function to retrieve active_scan_mask index

Add a function to retrieve the index of the active scan mask inside the
available scan masks array.

As in iio_scan_mask_match and iio_sanity_check_avail_scan_masks,
this function does not handle multi-long masks correctly.
It only checks the first long to be zero, and will use such mask
as a terminator even if there was bits set after the first long.

This should be fine since the available_scan_mask has already been
sanity tested using iio_sanity_check_avail_scan_masks.

See iio_scan_mask_match and iio_sanity_check_avail_scan_masks for
more details

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240731-ad7380-add-single-ended-chips-v2-2-cd63bf05744c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>