cxl/pci: Update comment

The existence of struct cxl_dev_id containing a single member is odd.
The comment made sense when I wrote it but could be clarified.

Update the comment and place it next to the odd looking structure.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20230426-cxl-fixes-v1-2-870c4c8b463a@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>