sfc: read back RX hash config from the NIC when querying it with ethtool -x

Ensures that we report the key and indirection table the NIC is using,
 rather than (if setting them failed earlier) what we wanted it to use.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>