net: pse-pd: pse_core: Add missing kdoc return description

Add missing kernel documentation return description.
This allows to remove all warning from kernel-doc test script.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://lore.kernel.org/r/20240423-fix_poe-v3-1-e50f32f5fa59@bootlin.com


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