net: phylink: update mac_config() documentation

A detail for mac_config() had been missed in the documentation for the
method - it is expected that the method will update the MAC to the
settings, rather than completely reprogram the MAC on each call.
Update the documentation for this method for this detail.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/phylink.h

@@ -149,6 +149,13 @@ int mac_link_state(struct net_device *ndev,
  *   configuration word. Nothing is advertised by the MAC. The MAC is
  *   responsible for reading the configuration word and configuring
  *   itself accordingly.
+ *
+ * Implementations are expected to update the MAC to reflect the
+ * requested settings - i.o.w., if nothing has changed between two
+ * calls, no action is expected.  If only flow control settings have
+ * changed, flow control should be updated *without* taking the link
+ * down.  This "update" behaviour is critical to avoid bouncing the
+ * link up status.
  */
 void mac_config(struct net_device *ndev, unsigned int mode,
 		const struct phylink_link_state *state);