staging:iio:adis16080: Perform sign extension

The adis16080 reports sample values in twos complement. So we need to perform a
sign extension on the result, otherwise negative values will be reported
incorrectly as large positive values.

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