iio:buffer: Wrong sized allocation of demux table elements.

The size of the allocation is currently set to the size of the pointer
rather than the structure we should actually be allocating.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reported-by:  <kbuild@01.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>