staging:iio:adis16260: Add scale for the inclination channel

While the inclination channel claims to support reading the scale the driver did
not implement this, so trying to read the scale results in a -EINVAL. This patch
fixes it.

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