cxgb4: restructure VF mgmt code

restructure the code which adds support for configuring
PCIe VF via mgmt netdevice. which was added by
commit 7829451c

 ("cxgb4: Add control net_device for
configuring PCIe VF")

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