net: pse-pd: Remove unused pse_ethtool_get_pw_limit function declaration

Removed the unused pse_ethtool_get_pw_limit() function declaration from
pse.h. This function was declared but never implemented or used,
making the declaration unnecessary.

Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Kyle Swenson <kyle.swenson@est.tech>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>