MIPS: Loongson: Add return value check for strict_strtoul()

cc1: warnings being treated as errors
arch/mips/loongson/common/env.c: In function 'prom_init_env':
arch/mips/loongson/common/env.c:49: error: ignoring return value of 'strict_strtol', declared with attribute warn_unused_result
arch/mips/loongson/common/env.c:50: error: ignoring return value of 'strict_strtol', declared with attribute warn_unused_result
arch/mips/loongson/common/env.c:51: error: ignoring return value of 'strict_strtol', declared with attribute warn_unused_result
arch/mips/loongson/common/env.c:52: error: ignoring return value of 'strict_strtol', declared with attribute warn_unused_result

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1762/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>