cxgb4vf: Forcefully link up virtual interfaces

The Virtual Interfaces are connected to an internal switch on the chip
which allows VIs attached to the same port to talk to each other even
when the port link is down.  As a result, we generally want to always
report a VI's link as being "up".

Based on the original work by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>