hv_netvsc: Check VF datapath when sending traffic to VF

The driver needs to check if the datapath has been switched to VF before
sending traffic to VF.

Signed-off-by: Long Li <longli@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>