eth: gianfar: migrate to new RXFH callbacks

eth: gianfar: migrate to new RXFH callbacks

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

Uniquely, this driver supports only the SET operation. It does not
support GET at all. The SET callback also always returns 0, even
tho it checks a bunch of conditions, and if my quick reading is
right, expects the user to insert filtering rules for given flow
type first? Long story short it seems too convoluted to easily
add the GET as part of the conversion.

Link: https://patch.msgid.link/20250613172751.3754732-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>