mlxsw: spectrum_router: Move multipath hash configuration to a bitmap

Currently, the multipath hash configuration is written directly to the
register payload. While this is OK for the two currently supported
policies, it is going to be hard to follow when more policies and more
packet fields are added.

Instead, set the required headers and fields in a bitmap and then dump
it to the register payload.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>