Fix #1328: Make error return codes negative values

Fix the bug where certain functions were returning meaningful return
and/or error code, where positive values of error codes were interfering
with meaningful return value. Error codes now have negative values as it
was originally intended but never implemented.

include/libopencm3/stm32/fdcan.h

@@ -723,28 +723,30 @@ struct fdcan_tx_buffer_element {
 #define FDCAN_FIFO_RXTS_MASK			0xFFFF
 
 
-/** FDCAN error return values
+/** @defgroup fdcan_error FDCAN error return values
+ * @{
  */
-enum fdcan_error {
+
 /** No error. Operation finished successfully */
-	FDCAN_E_OK,
+#define FDCAN_E_OK						0
 
 /** Value provided was out of range */
-	FDCAN_E_OUTOFRANGE,
+#define FDCAN_E_OUTOFRANGE				-1
 
 /** Timeout waiting for FDCAN block to accept INIT bit change */
-	FDCAN_E_TIMEOUT,
+#define FDCAN_E_TIMEOUT					-2
 
 /** Value provided was invalid (FIFO index, FDCAN block base address, length, etc.) */
-	FDCAN_E_INVALID,
+#define FDCAN_E_INVALID					-3
 
 /** Device is busy: Transmit buffer is full, unable to queue additional message or device
  * is outside of INIT mode and cannot perform desired operation. */
-	FDCAN_E_BUSY,
+#define FDCAN_E_BUSY					-4
 
 /** Receive buffer is empty, unable to read any new message */
-	FDCAN_E_NOTAVAIL
+#define FDCAN_E_NOTAVAIL				-5
-};
+
+/**@}*/
 
 /**@}*/