MIPS: BCM47XX: Use helpers for reading NVRAM content

Also drop some unneeded memset-s.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8661/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --cc arch/mips/bcm47xx/nvram.c
Simple merge