MIPS: BCM47XX: Simplify function looking for NVRAM entry

MIPS: BCM47XX: Simplify function looking for NVRAM entry

First of all it shouldn't modify copied NVRAM just to make sure it can
loop over all entries. It's enough to just compare current position
pointer with the end of buffer address.
Secondly buffer is guaranteed to be \0 ended, so we don't need strnchr.

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/10032/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>