ethtool: Don't check for RXFH fields conflict when no input_xfrm is requested

ethtool: Don't check for RXFH fields conflict when no input_xfrm is requested

The requirement of ->get_rxfh_fields() in ethtool_set_rxfh() is there to
verify that we have no conflict of input_xfrm with the RSS fields
options, there is no point in doing it if input_xfrm is not
supported/requested.

This is under the assumption that a driver that supports input_xfrm will
also support ->get_rxfh_fields(), so add a WARN_ON() to
ethtool_check_ops() to verify it, and remove the op NULL check.

This fixes the following error in mlx4_en, which doesn't support
getting/setting RXFH fields.
$ ethtool --set-rxfh-indir eth2 hfunc xor
Cannot set RX flow hash configuration: Operation not supported

Fixes: 72792461c8e8 ("net: ethtool: don't mux RXFH via rxnfc callbacks")
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250715140754.489677-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>