openvswitch: Change ENOENT return value to ENODEV in lookup_vport().

This reduces the number of valid "no such device" error values that
need special attention by the caller.

Userspace code will need to keep on checking for both ENODEV and
ENOENT as long as older kernel modules are around.

Signed-off-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>