staging:iio:ad7780: Mark channels as unsigned

The values reported by the AD7780 are unsigned with a binary offset:

	0x000000 is negative fullscale
	0x800000 is zeroscale
	0xffffff is positive fullscale

So mark the channel in the channel spec as unsigned rather than signed.

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