这个结构是定义一个USB串行驱动。它提供了所有的的信息USB系列核心代码的需要。如果函数指针定义,然后当TTY端口功能一致的时候USB系列核心代码会叫他们是所谓的天圆。如果他们不被调用，则为通用串行函数来代替。-This structure is defines a USB Serial driver.　It provides all of the information that the USB serial core code needs.　If the function pointers are defined, then the USB serial core code will call them when the corresponding tty port functions are called.　If they are not called, the generic serial function will be used instead.