net: mdio-mux: be compatible with parent buses which only support C45

After the mii_bus API conversion to a split read() / read_c45(), there
might be MDIO parent buses which only populate the read_c45() and
write_c45() function pointers but not the C22 variants.

We haven't seen these in the wild paired with MDIO multiplexers, but
Andrew points out we should treat the corner case.

Link: https://lore.kernel.org/netdev/4ccd7dc9-b611-48aa-865f-68d3a1327ce8@lunn.ch/Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231213152712.320842-3-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mdio/mdio-mux.c

@@ -214,8 +214,10 @@ int mdio_mux_init(struct device *dev,
 		snprintf(cb->mii_bus->id, MII_BUS_ID_SIZE, "%s-%x.%x",
 			 cb->mii_bus->name, pb->parent_id, v);
 		cb->mii_bus->parent = dev;
-		cb->mii_bus->read = mdio_mux_read;
-		cb->mii_bus->write = mdio_mux_write;
+		if (parent_bus->read)
+			cb->mii_bus->read = mdio_mux_read;
+		if (parent_bus->write)
+			cb->mii_bus->write = mdio_mux_write;
 		if (parent_bus->read_c45)
 			cb->mii_bus->read_c45 = mdio_mux_read_c45;
 		if (parent_bus->write_c45)