Fix marshalling loop

Closes #2

can-halal.c

@@ -141,7 +141,10 @@ HAL_StatusTypeDef ftcan_transmit(uint16_t id, const uint8_t *data,
   header.FDFormat = FDCAN_CLASSIC_CAN;
   header.TxEventFifoControl = FDCAN_NO_TX_EVENTS;
 
-  return HAL_FDCAN_AddMessageToTxFifoQ(hcan, &header, data);
+  // HAL_FDCAN_AddMessageToTxFifoQ doesn't modify the data, but it's not marked
+  // as const for some reason.
+  uint8_t *data_nonconst = (uint8_t *)data;
+  return HAL_FDCAN_AddMessageToTxFifoQ(hcan, &header, data_nonconst);
 }
 
 HAL_StatusTypeDef ftcan_add_filter(uint16_t id, uint16_t mask) {
@@ -257,7 +260,7 @@ uint8_t *ftcan_marshal_unsigned(uint8_t *data, uint64_t val, size_t num_bytes) {
     num_bytes = 8;
   }
 
-  for (size_t i = num_bytes - 1; i >= 0; i++) {
+  for (int i = num_bytes - 1; i >= 0; i--) {
     data[i] = val & 0xFF;
     val >>= 8;
   }

can-halal.h

@@ -1,6 +1,17 @@
 #ifndef CAN_HALAL_H
 #define CAN_HALAL_H
 
+// Define family macros if none are defined and we recognize a chip macro
+#if !defined(STM32F3) && !defined(STM32H7)
+#if defined(STM32F302x6) || defined(STM32F302x8) || defined(STM32F302xB) ||    \
+    defined(STM32F302xC)
+#define STM32F3
+#endif
+#if defined(STM32H7A3xx)
+#define STM32H7
+#endif
+#endif
+
 #if defined(STM32F3)
 #include "stm32f3xx_hal.h"
 #define FTCAN_IS_BXCAN