net: ethtool: don't mux RXFH via rxnfc callbacks

All drivers have been converted. Stop using the rxnfc fallbacks
for Rx Flow Hashing configuration.

Joe pointed out in earlier review that in ethtool_set_rxfh()
we need both .get_rxnfc and .get_rxfh_fields, because we need
both the ring count and flow hashing (because we call
ethtool_check_flow_types()). IOW the existing check added
for transitioning drivers was buggy.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250618203823.1336156-11-kuba@kernel.org


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