iio: adc: mcp3911: make use of the sign bit

The device supports negative values as well.

Fixes: 3a89b289 ("iio: adc: add support for mcp3911")
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220722130726.7627-2-marcus.folkesson@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>