mlxsw: spectrum_switchdev: Remove reference count from mid

Since there is a bitmap for the ports registered to each mid, there is no
need for a ref count, since it will always be the number of set bits in
this bitmap. Any check of the ref count was replaced with checking if the
bitmap is empty.

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>