Drivers: net: hyperv: Negotiate suitable ndis version for offload support

Ws2008R2 supports ndis_version 6.1 and 6.1 is the minimal version required
for various offloads. Negotiate ndis_version 6.1 when on ws2008r2.

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