tty: clarify that not all ttys have a class device

Commit 30004ac9 ("tty: add tty_struct->dev pointer to corresponding
device instance") added a struct device pointer field to struct
tty_struct which was populated with the corresponding tty class device
during initialisation.

Unfortunately, not all ttys have a class device (e.g. pseudoterminals
and serdev) in which case the device pointer will be set to NULL,
something which have bit driver authors over the years.

In retrospect perhaps this field should never have been added, but let's
at least document the current behaviour.
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210409073512.6876-1-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty.h

@@ -284,7 +284,7 @@ struct tty_operations;
 struct tty_struct {
 	int	magic;
 	struct kref kref;
-	struct device *dev;
+	struct device *dev;	/* class device or NULL (e.g. ptys, serdev) */
 	struct tty_driver *driver;
 	const struct tty_operations *ops;
 	int index;