MIPS: BCM63xx: Prevent second enet registration on BCM6338

This SoC has only one ethernet MAC, so prevent registration of a second one.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1482/


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