The goal is to stop setting and using tty->driver_data in devpts code.
It should be used solely by the driver's code, pty in this case.
Now driver_data are managed only in the pty driver. devpts_pty_new is
switched to accept what we used to dig out of tty_struct, i.e. device
node number and index.
This also removes a note about driver_data being set outside of the
driver.