ethtool: prevent null pointer dereference with NTUPLE set but no set_rx_ntuple

ethtool: prevent null pointer dereference with NTUPLE set but no set_rx_ntuple

This change is meant to prevent a possible null pointer dereference if
NETIF_F_NTUPLE is defined but the set_rx_ntuple function pointer is not.

The main motivation behind this patch is to eventually replace the ntuple
interfaces entirely with the network flow classifier interfaces.  This
allows the device drivers to maintain the ntuple check internally while
using the network flow classifier interface for setting up and displaying
rules.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>