sfc: don't report RX hash keys to ethtool when RSS wasn't enabled

If we failed to set up RSS on EF10 (e.g. because firmware declared
 RX_RSS_LIMITED), ethtool --show-nfc $dev rx-flow-hash ... should report
 no fields, rather than confusingly reporting what fields we _would_ be
 hashing on if RSS was working.

Fixes: dcb4123c

 ("sfc: disable RSS when unsupported")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>