ethtool: document PHY tunable callbacks

Add missing kdoc for phy tunable callbacks.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ethtool.h

@@ -410,6 +410,8 @@ struct ethtool_pause_stats {
  * @get_ethtool_phy_stats: Return extended statistics about the PHY device.
  *	This is only useful if the device maintains PHY statistics and
  *	cannot use the standard PHY library helpers.
+ * @get_phy_tunable: Read the value of a PHY tunable.
+ * @set_phy_tunable: Set the value of a PHY tunable.
  *
  * All operations are optional (i.e. the function pointer may be set
  * to %NULL) and callers must take this into account.  Callers must