mtd: ubi: prevent unauthorized users to access scan_peb sysfs entry

scan_peb is a sysfs entry used to scan the time stamp of all PEBs. If a
PEB's time stamp is older than a given threshold, it is scheduled for
scrubbing. When activated, it will cause intensive CPU usage until
scan is completed.
Today any user can access the scan_peb sysfs entry, this may cause a
security vulnerability for DoS attacks.
Fix this by removing the write permissions, leaving them only to admin
users.

Change-Id: I732196bd81de4c14fc3f999544907346c7ac0044
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>