net: sparx5: use is_port_rgmii() throughout

Now that we can check if a given port is an RGMII port, use it in the
following cases:

 - To set RGMII PHY modes for RGMII port devices.

 - To avoid checking for a SerDes node in the devicetree, when the port
   is an RGMII port.

 - To bail out of sparx5_port_init() when the common configuration is
   done.

Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
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-v5-3-fa8ba5dff732@microchip.com


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