can: usb_8dev: add LED trigger support

Add support for canbus activity led indicators on usb_8dev devices by
calling appropriate can_led functions.

These are only enabled when CONFIG_CAN_LEDS is Y, becomes no-op
otherwise.

Signed-off-by: Bernd Krumboeck <krumboeck@universalnet.at>
Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>