cxgb4: Report correct link speed for unsupported ones

When we get garbage from the firmware with weird Port Speeds,
etc. we should emit a warning regarding unsupported speeds rather than
use the bogus default of "10Mbps" which isn't even an option in the
firmware Port Information message

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>