drv/uart/stm32f4_ll_irq: only read USARTx->SR once in IRQ handler

It appears that reading the USART SR register clears some flags. In
certain cases, this would cause the interrupt handler to go into an
infinite loop.

By reading the SR register only once, we ensure that all flags are
seen and handled properly.

Issue: pybricks/support#304

lib/pbio/drv/uart/uart_stm32f4_ll_irq.c

@@ -185,13 +185,14 @@ void pbdrv_uart_flush(pbdrv_uart_dev_t *uart_dev) {
 void pbdrv_uart_stm32f4_ll_irq_handle_irq(uint8_t id) {
     pbdrv_uart_t *uart = &pbdrv_uart[id];
     USART_TypeDef *USARTx = uart->pdata->uart;
+    uint32_t sr = USARTx->SR;
 
-    if (LL_USART_IsEnabledIT_RXNE(USARTx) && LL_USART_IsActiveFlag_RXNE(USARTx)) {
+    if (LL_USART_IsEnabledIT_RXNE(USARTx) && sr & USART_SR_RXNE) {
         ringbuf_put(&uart->rx_buf, LL_USART_ReceiveData8(USARTx));
         process_poll(&pbdrv_uart_process);
     }
 
-    if (LL_USART_IsEnabledIT_TXE(USARTx) && LL_USART_IsActiveFlag_TXE(USARTx)) {
+    if (LL_USART_IsEnabledIT_TXE(USARTx) && sr & USART_SR_TXE) {
         LL_USART_TransmitData8(USARTx, uart->write_buf[uart->write_pos++]);
         // When all bytes have been written, wait for the Tx complete interrupt.
         if (uart->write_pos == uart->write_length) {
@@ -200,7 +201,7 @@ void pbdrv_uart_stm32f4_ll_irq_handle_irq(uint8_t id) {
         }
     }
 
-    if (LL_USART_IsEnabledIT_TC(USARTx) && LL_USART_IsActiveFlag_TC(USARTx)) {
+    if (LL_USART_IsEnabledIT_TC(USARTx) && sr & USART_SR_TC) {
         LL_USART_DisableIT_TC(USARTx);
         process_poll(&pbdrv_uart_process);
     }