sfc: falcon: refactor and document ef4_ethtool_get_rxfh_fields

sfc: falcon: refactor and document ef4_ethtool_get_rxfh_fields

The code had some rather odd control flow inherited from when it was
 shared with siena and ef10 before this driver was split out.
Simplify that for easier reading.
Also add a comment explaining why we return the values we do, since
 some Falcon documents and datasheets confusingly mention the part
 supporting 4-tuple UDP hashing.
(I couldn't find any record of exactly what was "broken" about the
 original Falcon A hash, I'm just trusting that falcon_init_rx_cfg()
 had a good reason for not using it.)

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20250710173213.1638397-1-edward.cree@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>