hv_netvsc: Add query for initial physical link speed

The physical link speed value will be reported by ethtool command.
The real speed is available from Windows 2016 host or later.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>