UBI: Init vol->reserved_pebs by assignment

`vol' is a newly allocated value by kzalloc. Initialize it by assignment
instead of `+='.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: e8d266cf8d24e52eac65bdab24afd6f4cfc0ad07
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>