net: pse-pd: Do not return EOPNOSUPP if config is null

net: pse-pd: Do not return EOPNOSUPP if config is null

For a PSE supporting both c33 and PoDL, setting config for one type of PoE
leaves the other type's config null. Currently, this case returns
EOPNOTSUPP, which is incorrect. Instead, we should do nothing if the
configuration is empty.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Fixes: d83e13761d5b ("net: pse-pd: Use regulator framework within PSE framework")
Link: https://patch.msgid.link/20240711-fix_pse_pd_deref-v3-1-edd78fc4fe42@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>