net: ethtool: require drivers to opt into the per-RSS ctx RXFH

RX Flow Hashing supports using different configuration for different
RSS contexts. Only two drivers seem to support it. Make sure we
uniformly error out for drivers which don't.

Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20250611145949.2674086-4-kuba@kernel.org


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