staging:iio:dac:ad5686: Check for negative values

Currently it is possible to write negative values to the ad5686's raw attribute.
This will cause undefined behaviour, so reject negative values.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>