can: gs_usb: GS_CAN_FLAG_OVERFLOW: make use of BIT()

This patch converts the driver to use BIT() to define flags.

Link: https://lore.kernel.org/all/20220309124132.291861-5-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>