net: dsa: stop calling dev_hold in dsa_slave_fdb_event

Now that we guarantee that SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE events have
finished executing by the time we leave our bridge upper interface,
we've established a stronger boundary condition for how long the
dsa_slave_switchdev_event_work() might run.

As such, it is no longer possible for DSA slave interfaces to become
unregistered, since they are still bridge ports.

So delete the unnecessary dev_hold() and dev_put().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>