cxgb4: Initialize mbox lock and list for mgmt dev

Initialize mbox lock and list for mgmt dev to avoid NULL pointer
dereference when cxgb_set_vf_mac is called.

And also allocate memory for private data while allocating mgmt
netdev.

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