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.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/iio.h

@@ -855,6 +855,24 @@ static inline const struct iio_scan_type
 	return &chan->scan_type;
 }
 
+/**
+ * iio_get_masklength - Get length of the channels mask
+ * @indio_dev: the IIO device to get the masklength for
+ */
+static inline unsigned int iio_get_masklength(const struct iio_dev *indio_dev)
+{
+	return indio_dev->masklength;
+}
+
+/**
+ * iio_for_each_active_channel - Iterated over active channels
+ * @indio_dev: the IIO device
+ * @chan: Holds the index of the enabled channel
+ */
+#define iio_for_each_active_channel(indio_dev, chan) \
+	for_each_set_bit((chan), (indio_dev)->active_scan_mask, \
+			 iio_get_masklength(indio_dev))
+
 ssize_t iio_format_value(char *buf, unsigned int type, int size, int *vals);
 
 int iio_str_to_fixpoint(const char *str, int fract_mult, int *integer,