staging: iio: cdc: ad7150: use value copy to shorten the line length

By using the copy of channel attribute, we can now make the lines short
enough to eliminate the checkpatch.pl problem:

    CHECK: Alignment should match open parenthesis

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>