qualcommax: qca_ppe: stop advertising RGMII on the SerDes ports

qca_ppe_phylink_get_caps() adds the four RGMII modes to ports 1 to 4, and
then copies the port's supported interfaces into config->pcs_interfaces, so
phylink is told both the MAC and the PCS carry RGMII there. Neither does.
qca_ppe_mac_link_up() and qca_ppe_mac_link_down() have no RGMII arm and
return from their default label; the uniphy PCS driver has no RGMII code at
all. Nor can the ports carry it: every board describes these four with a
pcs-handle into uniphy0, the SerDes block that driver serves, and
ppe_pcs_set_mux_hppe() has no arm for ports 1 to 3 while port 4's select
names uniphy0's PCS0 whatever the interface.

The failure that leaves is silent. phylink_link_up() calls
netif_carrier_on() whether or not the MAC callback did anything, and
mac_link_up() is void, so its early return is not noticed. It returns
before it opens PPE_PORT_BRIDGE_CTRL_TXMAC_EN, the gate the fabric needs
before it will hand the port a frame, and before it enables the port's MAC.
A board describing one of these ports as RGMII would read link up at its
negotiated speed and pass no traffic, with no message anywhere. Once the
mode is not advertised, phylink_validate() rejects it and
phylink_bringup_phy() fails the port with a warning instead.

No qualcommax board describes a PPE port as RGMII: the one rgmii-rxid in
the target's device trees is on a cascaded qca8337's CPU port, whose
PPE-side port is qsgmii. What still reaches a default label after this is
the CPU port's internal link, whose gate qca_ppe_port_enable() opens.

Tested on IPQ8074 (Xiaomi AX3600, kernel 6.18.44): the four user ports come
up at 1000, 1000, 100 and 1000 Mbit/s full duplex, and forwarding, offload
and dmesg are unchanged.

Fixes: 142104bb90 ("qualcommax: add PPE driver")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Julius Bairaktaris <julius@bairaktaris.de>
Link: https://github.com/openwrt/openwrt/pull/25084
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/qualcommax/files/drivers/net/ethernet/qualcomm/qca_ppe_main.c

@@ -946,14 +946,6 @@ static void qca_ppe_phylink_get_caps(struct dsa_switch *ds, int port,
 			  config->supported_interfaces);
 		__set_bit(PHY_INTERFACE_MODE_SGMII,
 			  config->supported_interfaces);
-		__set_bit(PHY_INTERFACE_MODE_RGMII,
-			  config->supported_interfaces);
-		__set_bit(PHY_INTERFACE_MODE_RGMII_ID,
-			  config->supported_interfaces);
-		__set_bit(PHY_INTERFACE_MODE_RGMII_RXID,
-			  config->supported_interfaces);
-		__set_bit(PHY_INTERFACE_MODE_RGMII_TXID,
-			  config->supported_interfaces);
 		break;
 	case 5 ... 6:
 		config->mac_capabilities =