UBIFS: do not change debugfs file position

This patch is a tiny improvement which removes few bytes of code.
UBIFS debugfs files are non-seekable and the file position is ignored,
so do not increase it in the write handler.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/debug.c

@@ -2828,7 +2828,6 @@ static ssize_t write_debugfs_file(struct file *file, const char __user *buf,
 	} else
 		return -EINVAL;
 
-	*ppos += count;
 	return count;
 }