can: gw: use struct canfd_frame as internal data structure

To prepare the CAN FD support this patch implements the first adaptions in
data structures for CAN FD without changing the current functionality.

Additionally some code at the end of this patch is moved or indented to
simplify the review of the next implementation step.
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

include/uapi/linux/can/gw.h

@@ -93,10 +93,11 @@ enum {
 
 /* CAN frame elements that are affected by curr. 3 CAN frame modifications */
 #define CGW_MOD_ID	0x01
-#define CGW_MOD_DLC	0x02
+#define CGW_MOD_DLC	0x02		/* contains the data length in bytes */
+#define CGW_MOD_LEN	CGW_MOD_DLC	/* CAN FD length representation */
 #define CGW_MOD_DATA	0x04
 
-#define CGW_FRAME_MODS 3 /* ID DLC DATA */
+#define CGW_FRAME_MODS 3 /* ID DLC/LEN DATA */
 
 #define MAX_MODFUNCTIONS (CGW_MOD_FUNCS * CGW_FRAME_MODS)