iio: core: add accessors 'masklength'

iio: core: add accessors 'masklength'

'masklength' is supposed to be an IIO private member. However, drivers
(often in trigger handlers) need to access it to iterate over the
enabled channels for example (there are other reasons). Hence, a couple
of new accessors are being added:

 * iio_for_each_active_channel() - Iterates over the active channels;
 * iio_get_masklength() - Get length of the channels mask.

The goal of these new accessors is to annotate 'masklength' as private
as soon as all drivers accessing it are converted to use the new
helpers.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240702-dev-iio-masklength-private-v1-1-98193bf536a6@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>