MIPS: FW: Use kstrtoul() to parse unsigned long from the fw environment

Fix some value corruptions with values that can't be represented in a
signed long.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8358/


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