can: gs_usb: add switchable termination support

The candleLight community is working on switchable termination support
for the candleLight firmware. As the the Linux CAN framework supports
switchable termination add this feature to the gs_usb driver.

Devices supporting the feature should set the
GS_CAN_FEATURE_TERMINATION and implement the
GS_USB_BREQ_SET_TERMINATION and GS_USB_BREQ_GET_TERMINATION control
messages.

For now the driver assumes for activated termination the standard
termination value of 120Ω.

Link: https://lore.kernel.org/all/20220923074114.662045-1-mkl@pengutronix.de
Link: https://github.com/candle-usb/candleLight_fw/issues/92
Link: https://github.com/candle-usb/candleLight_fw/pull/109
Link: https://github.com/candle-usb/candleLight_fw/pull/108


Cc: Daniel Trevitz <daniel.trevitz@wika.com>
Cc: Ryan Edwards <ryan.edwards@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>