mlxsw: Keep track of number of allocated RIFs

In order to expose number of RIFs as a resource, it is going to be handy
to have the number of currently-allocated RIFs as a single number.
Introduce such.

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