[ Upstream commit c3152ec4 ]

We assume in the bcm_sf2 driver that the DSA master network device
supports ethtool_ops::{get,set}_wol operations, which is not a given.
Avoid de-referencing potentially non-existent function pointers and
check them as we should.

Fixes: 96e65d7f ("net: dsa: bcm_sf2: add support for Wake-on-LAN")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>