mlxsw: spectrum_router: Pass argument explicitly

Instead of relying on the LPM tree to be assigned to the virtual router
before binding the two, lets pass it explicitly.

This will later allow us to return upon binding error instead of having
to perform a rollback of the assignment.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>