stm32/uart: Clear overrun error flag after reading RX data register.

On MCUs other than F4 the ORE (overrun error) flag needs to be cleared
independently of clearing RXNE, even though both are wired to trigger the
same RXNE IRQ.  In the case that an overrun occurred it's necessary to
explicitly clear the ORE flag or else the RXNE interrupt will keep firing.

ports/stm32/uart.c

@@ -597,7 +597,9 @@ int uart_rx_char(pyb_uart_obj_t *self) {
     } else {
         // no buffering
         #if defined(STM32F0) || defined(STM32F7) || defined(STM32L4) || defined(STM32H7)
-        return self->uartx->RDR & self->char_mask;
+        int data = self->uartx->RDR & self->char_mask;
+        self->uartx->ICR = USART_ICR_ORECF; // clear ORE if it was set
+        return data;
         #else
         return self->uartx->DR & self->char_mask;
         #endif
@@ -722,6 +724,7 @@ void uart_irq_handler(mp_uint_t uart_id) {
                 // only read data if room in buf
                 #if defined(STM32F0) || defined(STM32F7) || defined(STM32L4) || defined(STM32H7)
                 int data = self->uartx->RDR; // clears UART_FLAG_RXNE
+                self->uartx->ICR = USART_ICR_ORECF; // clear ORE if it was set
                 #else
                 int data = self->uartx->DR; // clears UART_FLAG_RXNE
                 #endif