cxgb4: add skeleton for ethtool n-tuple filters

Allocate and manage resources required for ethtool n-tuple filters.
Also fetch the HASH filter region size and calculate nhash entries.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>