tty: Fix extra "not" in TTY_DRIVER_REAL_RAW description

TTY_DRIVER_REAL_RAW flag (which is always set for e.g. serial ports)
documentation says that driver must always set special character
handling flags in certain conditions.

However, as the following sentence makes clear, what is actually
intended is the opposite.

Fix that by removing the unintended double negation.

Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Link: https://lore.kernel.org/r/20211027102124.3049414-1-anssi.hannula@bitwise.fi
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: anssih

include/linux/tty_driver.h

@@ -360,7 +360,7 @@ static inline void tty_set_operations(struct tty_driver *driver,
  * 	Used for PTY's, in particular.
  * 
  * TTY_DRIVER_REAL_RAW --- if set, indicates that the driver will
- * 	guarantee never not to set any special character handling
+ * 	guarantee never to set any special character handling
  * 	flags if ((IGNBRK || (!BRKINT && !PARMRK)) && (IGNPAR ||
  * 	!INPCK)).  That is, if there is no reason for the driver to
  * 	send notifications of parity and break characters up to the