auxdisplay: hd44780: Fix oops on module unloading

Fixes: 718e05ed ("auxdisplay: Introduce hd44780_common.[ch]")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/CAHp75VfKyqy+vM0XkP9Yb+znGOTVT4zYCRY3A3nQ7C3WNUVN0g@mail.gmail.com/


Reported-By: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[added Link, Fixes, Cc stable tags, edited message]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>