MIPS: BCM47XX: fix detection for some boards

When a nvram reset was performed from CFE, it sometimes does not
contain the productid value in nvram, but it still contains
hardware_version.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6316/