mtd: ubi: reduce messages during scrub all PEBs

A daemon in userspace is periodically activating scrub-all PEBs sysfs
trigger. The scrubbing process causes extensive logging.
This patch reduces the logging during the scrubbing process.

Change-Id: I1c6396b54e39ea7d183e9d5e609c9632d610d85f
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>