tty: Document required behavior of tty driver close()

If the tty driver open() fails, the tty driver close() is still
called during the resultant tty release.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty_driver.h

@@ -40,6 +40,7 @@
  * void (*close)(struct tty_struct * tty, struct file * filp);
  *
  * 	This routine is called when a particular tty device is closed.
+ *	Note: called even if the corresponding open() failed.
  *
  *	Required method.
  *