iio: buffer: Coalesce adjacent demux table entries

When copying multiple multiple samples that are adjacent in both the source as
well as the destination buffer, instead of creating a new demux table entry for
each sample just increase the length of the previous entry by the size of the
new sample. This makes the demuxing process slightly more efficient.

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