udf: restore UDFFS_DEBUG to being undefined by default

Commit 706047a7, "udf: Fix compilation
warnings when UDF debug is on" inadvertently (I assume) enabled
debugging messages by default for UDF.  This patch disables them again.

Signed-off-by: Paul Collins <paul@ondioline.org>
Signed-off-by: Jan Kara <jack@suse.cz>