fs: ubifs: add ubi device index to trace

When running with multiple ubi mounted file systems, there is no way to
distinguish between messages from the different mounted fs. The device
index helps distinguishing between the mounted fs messages while
debugging.

Change-Id: I8f16af689198c643ec2c82e77a26818e583b4c29
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>