iio: light: si1145: use new '.masklength' accessors

Make use of iio_get_masklength) and iio_for_each_active_channel() to
access '.masklength' so it can be annotated as __private when there
are no more direct users of it.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240718-dev-iio-masklength-private2-v1-15-8e12cd042906@analog.com


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