MIPS: BCM47XX: Use strnchr to avoid reading out of the buffer

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/8662/


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