Hyper-V: hn: use VF's capabilities when it is attached

Current code in hn/netvsc tries to merge (logical AND) VF and
its own capability bits when a VF is attached. This results in
losing some key VF features, especially in tx path. For example,
the VF's txcsum, rxcsum or tso bits could be lost if any of
these are not in hn/netvsc's own capablility field.

Actually when VF is attached, hn just needs to use VF's caps
as all the tx packets would be forwarded to the VF interface.
Fix this problem by doing so.

Reported by: whu
Tested by: whu
MFC after: 3 days
Sponsored by: Microsoft