mlxsw: spectrum_router: Replace if statement with a switch statement

The code was written when only two multipath hash policies were present,
so the if statement was sufficient. The next patch and future patches
are going to add support for more policies, so move to a switch
statement.

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