hw/char/stm32l4x5_usart: Fix memory corruption by adding correct class_size

hw/char/stm32l4x5_usart: Fix memory corruption by adding correct class_size

"make check-qtest-aarch64" recently started failing on FreeBSD builds,
and valgrind on Linux also detected that there is something fishy with
the new stm32l4x5-usart: The code forgot to set the correct class_size
here, so the various class_init functions in this file wrote beyond
the allocated buffer when setting the subc->type field.

Fixes: 4fb37aea7e ("hw/char: Implement STM32L4x5 USART skeleton")
Message-ID: <20240429075908.36302-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>