net: pse-pd: Use EOPNOTSUPP error code instead of ENOTSUPP

ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP as reported by
checkpatch script.

Fixes: 18ff0bcd ("ethtool: add interface to interact with Ethernet Power Equipment")
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/20240610083426.740660-1-kory.maincent@bootlin.com


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