datapath-windows: Allow encapsulation if source is bridge-internal port.

It has been observed that when userspace generates and executes that
packet, the source port of such a packet is set to the bridge-internal
port. Currently, we allow encapsulation only if the source port is a VIF
port or no port. We relax the check in this patch.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>