eth: sfc: migrate to new RXFH callbacks

eth: sfc: migrate to new RXFH callbacks

Migrate to new callbacks added by commit 9bb00786fc61 ("net: ethtool:
add dedicated callbacks for getting and setting rxfh fields").

This driver's RXFH config is read only / fixed so the conversion
is purely factoring out the handling into a helper. One thing of
note that this is one of the two drivers which pays attention to
rss_context.

Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20250618203823.1336156-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>