ethtool: Fix filter addition when caching n-tuple filters

We can allow a filter to be added successfully to the underlying
hardware, but still return an error if the cached list memory
allocation fails.  This patch fixes that condition.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>