mlxsw: spectrum: Avoid possible NULL pointer dereference

In case we got an FDB notification for a port that doesn't exist we
execute an FDB entry delete to prevent it from re-appearing the next
time we poll for notifications.

If the operation failed we would trigger a NULL pointer dereference as
'mlxsw_sp_port' is NULL.

Fix it by reporting the error using the underlying bus device instead.

Fixes: 12f1501e

 ("mlxsw: spectrum: remove FDB entry in case we get unknown object notification")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>