sfc: check RSS is active for filter insert

For some firmware variants - specifically 'capture packed stream' - RSS
filters are not valid. We must check if RSS is actually active rather
than merely enabled.

Fixes: 42356d9a ("sfc: support RSS spreading of ethtool ntuple filters")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>