iio: magnetometer: rm3100-core: make use of iio_get_masklength()

Use iio_get_masklength() 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-v1-18-8e12cd042906@analog.com


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