mlxsw: spectrum_router: Apply user-defined multipath hash seed

When Spectrum machines compute hash for the purposes of ECMP routing, they
use a seed specified through RECR_v2 (Router ECMP Configuration Register).
Up until now mlxsw computed the seed by hashing the machine's base MAC.
Now that we can optionally have a user-provided seed, use that if possible.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-4-petrm@nvidia.com


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