iio: proximity: sx9500: Fix proximity value

Because of the ABI confusion proximity value exposed by SX9500
was inverted.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Vlad Dogaru <vlad.dogaru@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>