staging:iio:ad7793: Mark channels as unsigned

The values reported by the AD7793 are unsigned.
	In uniploar mode:
			0x000000 is zeroscale
			0xffffff is fullscale
	In bipolar mode:
			0x000000 is negative fullscale
			0x800000 is zeroscale
			0xffffff is positive fullscale

In bipolar mode there is a binary offset, but the values are still unsigned.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>