mlxsw: spectrum_acl: Make block arg const where appropriate

There are couple of places where block pointer as a function argument
can be const. So make those const.

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