mlxsw: spectrum_router: Configure default routing priority

When routing ip packets, the kernel is setting the SKB's priority
based on the tos field of the packet.
Imitate this behavior in the mlxsw router, having the internal
switch priority of a routed packet determined according to its DS
field.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>