cxgb4/cxgb4vf: Link management changes

1) Speed should be supported by Physical Port Capabilities.
2) report Forward Error Correction mode which are available.
3) Added few comments.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>