can: gs_usb: update GS_CAN_FEATURE_IDENTIFY documentation

In the binary interface a feature bit might have a corresponding mode
bit (of the same value).

The GS_CAN_FEATURE_IDENTIFY feature doesn't come with a mode. Document
this, to avoid gaps when adding more features/modes later.

Link: https://lore.kernel.org/all/20220309124132.291861-10-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/gs_usb.c

@@ -93,6 +93,7 @@ struct gs_device_config {
 #define GS_CAN_MODE_TRIPLE_SAMPLE BIT(2)
 #define GS_CAN_MODE_ONE_SHOT BIT(3)
 #define GS_CAN_MODE_HW_TIMESTAMP BIT(4)
+/* GS_CAN_FEATURE_IDENTIFY BIT(5) */
 
 struct gs_device_mode {
 	__le32 mode;