MIPS: BCM47XX: return error when init of nvram failed

This makes it possible to handle the case of not being able to read the
nvram ram. This could happen when the code searching for the specific
flash chip have not run jet.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4740/
Signed-off-by: John Crispin <blogic@openwrt.org>