can: gs_usb: document the PAD_PKTS_TO_MAX_PKT_SIZE feature

The widely used open source firmware candleLight supports padding the
USB packets to max packet size to improve performance on Windows
systems.

This patch documents the bit.

Link: https://lore.kernel.org/all/20220309124132.291861-12-mkl@pengutronix.de
Link: https://github.com/candle-usb/candleLight_fw/pull/7
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>