mlxsw: spectrum_switchdev: Only query FDB notifications when necessary

The driver periodically queries the device for FDB notifications (e.g.,
learned, aged-out) in order to update the bridge driver. These
notifications can only be generated when bridges are offloaded to the
device.

Avoid unnecessary queries by starting to query upon installation of the
first bridge and stop querying upon removal of the last bridge.

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