tty_driver: Update required method documentation

 * 	This routine is called by the kernel to write a series of

 * 	characters to the tty device.  The characters may come from

 * 	user space or kernel space.  This routine will return the

 *	number of characters actually accepted for writing.  This

 *	routine is mandatory.

 *	number of characters actually accepted for writing.

 *

 *	Optional: Required for writable devices.

 *

 * 	This routine notifies the tty driver that it should hangup the

 * 	tty device.

 *

 *	Required:

 *	Optional:

 *

 * void (*break_ctl)(struct tty_stuct *tty, int state);

 *