iio: proximity: as3935: fix buffer stack trashing

Buffer wasn't of a valid size to allow the timestamp, and correct padding.
This patchset also moves the buffer off the stack, and onto the heap.

Cc: george.mccollister@gmail.com
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>