After an initial link up the CAN device is in ERROR-ACTIVE mode. Due
to a missing CAN_STATE_STOPPED in gs_can_close() it doesn't change to
STOPPED after a link down:

| ip link set dev can0 up
| ip link set dev can0 down
| ip --details link show can0
| 13: can0: <NOARP,ECHO> mtu 16 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 10
|     link/can  promiscuity 0 allmulti 0 minmtu 0 maxmtu 0
|     can state ERROR-ACTIVE restart-ms 1000

Add missing assignment of CAN_STATE_STOPPED in gs_can_close().

Cc: stable@vger.kernel.org
Fixes: d08e973a ("can: gs_usb: Added support for the GS_USB CAN devices")
Link: https://lore.kernel.org/all/20230718-gs_usb-fix-can-state-v1-1-f19738ae2c23@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>