cxgb4: expose stats fetched from firmware via debugfs

Expose stats obtained from firmware via debugfs. These stats can't
be part of ethtool -S because the slow firmware mailbox can cause
packet drops under heavy load.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>