update AU1000 get_ethernet_addr()

Update AU1000 get_ethernet_addr().
Three functions were brought together in one.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>