iio: proximity: as3935: fix as3935_write

AS3935_WRITE_DATA macro bit is incorrect and the actual write
sequence is two leading zeros.

Cc: George McCollister <george.mccollister@gmail.com>
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>