iio: adc: xilinx: Fix VREFN sign
iio: adc: xilinx: Fix VREFN sign

The VREFN channel is bipolar, not unipolar. Small negative values do
occur (e.g., -1mV), and unsigned conversion maps them incorrectly to
large positive values (about +1V), so fix this.

Signed-off-by: Thomas Betker <5832886f93ec934ed3ea3b4687be7c55600d6eaf@rohde-schwarz.com>
Cc: <d96e5b2dd07c1733b2b013bd82922c22baade37d@vger.kernel.org>
Signed-off-by: Jonathan Cameron <09f65b71b7655725897b2fd41a09a0cefe2e1ace@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs