staging:iio: scrap scan_count and ensure all drivers use active_scan_mask

Obviously drivers should only use this for pushing to buffers.
They need buffer->scan_mask for pulling from them post demux.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>