iio: buffer: Fix demux table creation

When creating the demux table we need to iterate over the selected scan mask for
the buffer to get the samples which should be copied to destination buffer.
Right now the code uses the mask which contains all active channels, which means
the demux table contains entries which causes it to copy all the samples from
source to destination buffer one by one without doing any demuxing.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org