net: sparx5: only return PCS for modes that require it

The RGMII ports have no PCS to configure. Make sure we only return the
PCS for port modes that require it.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20241220-sparx5-lan969x-switch-driver-4-v5-5-fa8ba5dff732@microchip.com


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