staging:iio:cdc:ad7150: Add copyright notice given substantial changes.

It seems to me that the changes made to get this ready to move out of
staging are substantial enough to warant a copyright notice addition.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-23-jic23@kernel.org