net: RPS: Make hardware-accelerated RFS conditional on NETIF_F_NTUPLE

For testing and debugging purposes it is useful to be able to disable
hardware acceleration of RFS without disabling RFS altogether.  Since
this is a similar feature to 'n-tuple' flow steering through the
ethtool API, test the same feature flag that controls that.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>