cxgb4vf: define get_fecparam ethtool callback

Add support to new ethtool operation get_fecparam to
fetch FEC parameters.

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