MIPS: Octeon: device_tree_init: don't fill mac if already set

Don't fill MAC address if it's already set. This allows DTB to
override the bootinfo.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12589/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>