hw/char/stm32l4x5_usart.c: Enable USART ACK bit response

hw/char/stm32l4x5_usart.c: Enable USART ACK bit response

SW modifying USART_CR1 TE bit should cuase HW to respond by altering
USART_ISR TEACK bit, and likewise for RE and REACK bit.

This resolves some but not all issues necessary for the official STM USART
HAL driver to function as is.

Fixes: 87b77e6e01ca ("hw/char/stm32l4x5_usart: Enable serial read and write")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2540
Signed-off-by: Jacob Abrams <satur9nine@gmail.com>
Message-id: 20240911043255.51966-1-satur9nine@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 6cce0dcc6f7aaaeb7f17577776da510b04f67c99)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>