hv_netvsc: Allow scatter-gather feature to be tunable

In a previous patch, the NETIF_F_SG was missing after the code changes.
That caused the SG feature to be "fixed". This patch includes it into
hw_features, so it is tunable again.

Fixes: 23312a3b

 ("netvsc: negotiate checksum and segmentation parameters")
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>