cxgb4: fix memory leak in init_one()

Free up mbox_log allocated for PF0 to PF3.

Fixes: 7829451c ("cxgb4: Add control net_device for configuring PCIe VF")
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>