MIPS: BCM47XX: Make sure NVRAM buffer ends with \0

This will simplify reading its contents.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Hante Meuleman <meuleman@broadcom.com>
Cc: Ian Kent <raven@themaw.net>
Patchwork: https://patchwork.linux-mips.org/patch/10031/


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