staging:iio:adis16400: Preallocate transfer message

Similar to like we already did for the generic adis library preallocate and
pre-construct the SPI transfer message for the adis16400. For devices which do
not support burst mode sampling does not differ from other adis devices and so
we use the generic functions of the adis library in this case. In burst mode we
can only sample all channels at once, so use the IIO cores demux facility
instead of doing this manually.

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