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>