iio: buffer: initialize masklength accumulator to 0

iio: buffer: initialize masklength accumulator to 0

Since masklength is marked as [INTERN], no drivers should assign it and
the value will always be 0. Therefore, the local ml accumulator variable
in iio_buffers_alloc_sysfs_and_mask() will always start out as 0.

This changes the code to explicitly set ml to 0 to make it clear that
drivers should not be trying to override the masklength field.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240425-b4-iio-masklength-cleanup-v1-3-d3d16318274d@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>