We use transformation rather than accessors as virtually ever driver
implements SIOCGIFMEDIA and all would have to be touched.
Keep the code readable by always performing copies and (possiably no-op)
transforms.
Reviewed by: jhb, kib
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14996