can: gs_usb: add HW timestamp mode bit

Newer versions of the widely used open source firmware candleLight
support hardware timestamps. The support is activated by setting the
GS_CAN_MODE_HW_TIMESTAMP in the GS_USB_BREQ_MODE request.

Although timestamp support is not yet supported by this driver, add
the missing bit for documentation purpose.

Link: https://lore.kernel.org/all/20220309124132.291861-9-mkl@pengutronix.de
Link: https://github.com/candle-usb/candleLight_fw/commit/44431f4a4354a878fbd15b273bf04fce1dcdff7e
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>