can: gs_usb: document the USER_ID feature

The widely used open source firmware candleLight has optional support
for reading/writing of an user defined value into the device's flash.

This is indicated by the GS_CAN_FEATURE_USER_ID feature. The
corresponding request are GS_USB_BREQ_GET_USER_ID and
GS_USB_BREQ_SET_USER_ID.

This patch documents these values.

Link: https://lore.kernel.org/all/20220309124132.291861-11-mkl@pengutronix.de
Link: https://github.com/candle-usb/candleLight_fw/commit/1453d70dc9a9d98ac254893ba5114b8e826e0e39
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>