hyperv: Fix the NETIF_F_SG flag setting in netvsc

SG mode is not currently supported by netvsc, so remove this flag for now.
Otherwise, it will be unconditionally enabled by commit ec5f0615


    "Kill link between CSUM and SG features"
Previously, the SG feature is disabled because CSUM is not set here.

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>