net: mscc: ocelot: fix NULL pointer on LAG slave removal

lag_upper_info may be NULL on slave removal.

Fixes: dc96ee37 ("net: mscc: ocelot: add bonding support")
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>