vxlan: correct typo in call to unregister_netdevice_queue

By inspection, this appears to be a typo.  The gating comparison
involves vxlan->dev rather than dev.  In fact, dev is the iterator in
the preceding loop above but it is actually constant in the 2nd loop.

Use of dev seems to be a bad cut-n-paste from the prior call to
unregister_netdevice_queue.  Change dev to vxlan->dev, since that is
what is actually being checked.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>