cxgb4: add user manipulation of the RSS table

Implement the get_rxnfc, get_rxfh_indir, and set_rxfh_indir ethtool
methods for user manipulation of the RSS table.  Besides the methods
themselves the rest of the changes here store, initialize, and write
the table contents.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>