cxgb4/cxgb4vf: support for XLAUI Port Type

Add support for new Backplane XLAUI port type.

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