cxgb4: Support for get_ts_info ethtool method

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Atul Gupta <atul.gupta@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>