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.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>