cxgb4: do not set needs_free_netdev for mgmt dev's

Do not set 'needs_free_netdev' as we do call free_netdev
for mgmt net devices, doing both hits BUG_ON.

Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>