fixup: force error if CAN_DEVICE_DEFAULT_LOOP_DELAY not defined

mandatory for bitrate switching and HW related, so cannot have a default
value

sys/can/device.c

@@ -35,14 +35,20 @@
 #define ENABLE_DEBUG 0
 #include "debug.h"
 
+#ifdef MODULE_FDCAN
+/**
+ * The loop delay in CAN, especially in CAN FD with bitrate switching, affects synchronization due to increased data rates.
+ * The unit is nanoseconds.
+ */
+#ifndef CAN_DEVICE_DEFAULT_LOOP_DELAY
+#error "CAN_DEVICE_DEFAULT_LOOP_DELAY must be defined in the application's Makefile. This property can be found in the datasheet of the CAN transceiver in use. The unit is nanoseconds."
+#endif /* CAN_DEVICE_DEFAULT_LOOP_DELAY */
+#endif /* MODULE_FDCAN */
+
 #ifndef CAN_DEVICE_MSG_QUEUE_SIZE
 #define CAN_DEVICE_MSG_QUEUE_SIZE 64
 #endif
 
-#ifndef CAN_DEVICE_DEFAULT_LOOP_DELAY
-#define CAN_DEVICE_DEFAULT_LOOP_DELAY 200   /* nanoseconds */
-#endif
-
 #ifdef MODULE_CAN_PM
 #define CAN_DEVICE_PM_DEFAULT_RX_TIMEOUT (10 * US_PER_SEC)
 #define CAN_DEVICE_PM_DEFAULT_TX_TIMEOUT (2 * US_PER_SEC)