datapath: Report ifindex of 0 if vport doesn't have one.

datapath: Report ifindex of 0 if vport doesn't have one.

If a vport is a virtual device then it doesn't have a system ifindex.
We currently return the ifindex of the bridge device in this situation
but that's somewhat misleading, so this replaces it with 0.  Nothing
actually reads the ifindex for devices other than the bridge device,
so this doesn't have a functional change.

Suggested-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>