iio: add kernel-doc for field @owner

When building kernel documentation sphinx emits the following warning

	warning: No description found for parameter 'owner'

Add description for struct member 'owner'.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>