mlxsw: spectrum: Remove misuses of private header file

The spectrum_router.h header file is private to the router block and
should only be included by direct consumers of it, such as dpipe and the
multicast routing code.

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