jca@ points out that the field I split in iwx(4) Rx descriptors is in a union.

jca@ points out that the field I split in iwx(4) Rx descriptors is in a union.
Keep semantics of the field intact by wrapping the split values in struct { }.
Again no size change, because of union semantics.

diff from jca@ with request to commit if ok, thanks!
build-tested again + ok tb@