ubi: Simplify bool conversion

./drivers/mtd/ubi/build.c:1261:33-38: WARNING: conversion to bool not needed here

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4061
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>