eth: thunder: migrate to new RXFH callbacks

Migrate to new callbacks added by commit 9bb00786 ("net: ethtool:
add dedicated callbacks for getting and setting rxfh fields").

The driver has no other RXNFC functionality so the SET callback can
be now removed.

Link: https://patch.msgid.link/20250617014555.434790-5-kuba@kernel.org


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