ubi: Fix a mistake in comment

Fixes: 2a734bb8 ("UBI: use debugfs for the extra checks knobs")
There is a mistake in docstrings, it should be ubi_debugfs_exit_dev
instead of dbg_debug_exit_dev.
Signed-off-by: Kai Song <songkai01@inspur.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/debug.c

@@ -562,7 +562,7 @@ int ubi_debugfs_init_dev(struct ubi_device *ubi)
 }
 
 /**
- * dbg_debug_exit_dev - free all debugfs files corresponding to device @ubi
+ * ubi_debugfs_exit_dev - free all debugfs files corresponding to device @ubi
  * @ubi: UBI device description object
  */
 void ubi_debugfs_exit_dev(struct ubi_device *ubi)