net: ethtool: pse-pd: Fix possible null-deref

Fix a possible null dereference when a PSE supports both c33 and PoDL, but
only one of the netlink attributes is specified. The c33 or PoDL PSE
capabilities are already validated in the ethnl_set_pse_validate() call.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20240705184116.13d8235a@kernel.org/
Fixes: 4d18e3dd ("net: ethtool: pse-pd: Expand pse commands with the PSE PoE interface")
Link: https://patch.msgid.link/20240711-fix_pse_pd_deref-v3-2-edd78fc4fe42@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>