sfc: Add support for RX flow hash control

Allow ethtool to query the number of RX rings, the fields used in RX
flow hashing and the hash indirection table.

Allow ethtool to update the RX flow hash indirection table.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>