Documentation: net: dsa: Cut set_addr() documentation

This is not supported anymore, devices needing a MAC address
just assign one at random, it's just a driver pecularity.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>