Fix STM32F2XX USART data register readout

Fix issue where the data register may be overwritten by next character
reception before being read and returned.

Signed-off-by: Olivier Hériveaux <olivier.heriveaux@ledger.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211128120723.4053-1-olivier.heriveaux@ledger.fr
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/char/stm32f2xx_usart.c

@@ -103,10 +103,11 @@ static uint64_t stm32f2xx_usart_read(void *opaque, hwaddr addr,
         return retvalue;
     case USART_DR:
         DB_PRINT("Value: 0x%" PRIx32 ", %c\n", s->usart_dr, (char) s->usart_dr);
+        retvalue = s->usart_dr & 0x3FF;
         s->usart_sr &= ~USART_SR_RXNE;
         qemu_chr_fe_accept_input(&s->chr);
         qemu_set_irq(s->irq, 0);
-        return s->usart_dr & 0x3FF;
+        return retvalue;
     case USART_BRR:
         return s->usart_brr;
     case USART_CR1: