net: ethtool: add dedicated callbacks for getting and setting rxfh fields

We mux multiple calls to the drivers via the .get_nfc and .set_nfc
callbacks. This is slightly inconvenient to the drivers as they
have to de-mux them back. It will also be awkward for netlink code
to construct struct ethtool_rxnfc when it wants to get info about
RX Flow Hash, from the RSS module.

Add dedicated driver callbacks. Create struct ethtool_rxfh_fields
which contains only data relevant to RXFH. Maintain the names of
the fields to avoid having to heavily modify the drivers.

For now support both callbacks, once all drivers are converted
ethtool_*et_rxfh_fields() will stop using the rxnfc callbacks.

Link: https://patch.msgid.link/20250611145949.2674086-5-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>