staging:iio:ad799x: Preallocate sample buffer

Avoid allocating and freeing the sample buffer for each transfer. Instead
allocate it once when we start sampling. Also pre-compute the number of bytes we
need to transfer in the same way.

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