mdio-netlink: Restore compatibility with kernels older than 5.8

The fixed commit added support for kernels newer than 5.7, but broke
support for older kernels. This change backports the C45 wrappers
needed to make those versions viable again.

Fixes: 6cfdfd0

kernel/compat.h

@@ -5,6 +5,30 @@
 
 #include <linux/version.h>
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,8,0)
+#include <linux/device.h>
+#include <linux/phy.h>
+
+static inline u32 mdiobus_c45_addr(int devad, u16 regnum)
+{
+	return MII_ADDR_C45 | devad << MII_DEVADDR_C45_SHIFT | regnum;
+}
+
+static inline int __mdiobus_c45_read(struct mii_bus *bus, int prtad, int devad,
+				     u16 regnum)
+{
+	return __mdiobus_read(bus, prtad, mdiobus_c45_addr(devad, regnum));
+}
+
+static inline int __mdiobus_c45_write(struct mii_bus *bus, int prtad, int devad,
+				      u16 regnum, u16 val)
+{
+	return __mdiobus_write(bus, prtad, mdiobus_c45_addr(devad, regnum),
+			       val);
+}
+
+#endif	/* < 5.8.0 */
+
 #if LINUX_VERSION_CODE < KERNEL_VERSION(5,7,0)
 #include <linux/device.h>
 #include <linux/phy.h>
@@ -21,7 +45,7 @@ static inline int _mdio_find_bus_match(struct device *dev, const void *data)
 {
 	return dev->parent && device_match_name(dev->parent, data);
 }
-#endif
+#endif	/* < 5.4.0 */
 
 /**
  * mdio_find_bus - Given the name of a mdiobus, find the mii_bus.
@@ -46,6 +70,6 @@ static inline struct mii_bus *mdio_find_bus(const char *mdio_name)
 	put_device(d);
 	return p ? to_mii_bus(p) : NULL;
 }
-#endif
+#endif	/* < 5.7.0 */
 
 #endif /* _COMPAT_H_ */