staging:iio:ad7476: Avoid alloc/free for each sample in buffered mode

The ad7476 driver has only support for 1 channel ADCs. So the upper limit for
the buffer size is the size of one sample plus the size of the timestamp.
Preallocate a buffer large enough to hold this to avoid having to allocate and
free a new buffer for each sample being captured.

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