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.

Approved by: re (gjb)
Reported by: whu
Tested by: whu
Sponsored by: Microsoft

(cherry picked from commit 289ba6b870f79d8dbd9e8022c4fa23469be11a73)
(cherry picked from commit c9b4782ec336c122d8ede81be0823969e0748332)