canio: mimxrt10xx: Fix CAN frame reception

This code clears the "frame available" flag, which allows the CPU to read
and serve the next entry of the Rx FIFO.

Without that, the CPU would never advance in the Rx FIFO and will serve
the oldest mailbox over and over, until the Rx FIFO overflows.

Author: SamantazFox

ports/mimxrt10xx/common-hal/canio/Listener.c

@@ -115,6 +115,10 @@ mp_obj_t common_hal_canio_listener_receive(canio_listener_obj_t *self) {
         mp_raise_OSError(MP_EIO);
     }
 
+    // We've read from the FIFO, clear the "frame available" flag, which
+    // allows the CPU to serve the next FIFO entry
+    FLEXCAN_ClearMbStatusFlags(self->can->data->base, (uint32_t)kFLEXCAN_RxFifoFrameAvlFlag);
+
     canio_message_obj_t *message = m_new_obj(canio_message_obj_t);
     if (!mimxrt_flexcan_frame_to_canio_message_obj(&rx_frame, message)) {
         mp_raise_ValueError(MP_ERROR_TEXT("Unable to receive CAN Message: missing or malformed flexcan frame"));