staging:iio:ad7887: Preallocate sample buffer

We know that the sample buffer will at most need to hold two 16 bit samples and
the 64 bit aligned 64 bit timestamp. Preallocate a buffer large enough to hold
this instead of allocating and freeing it each time a sample is read.

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