can: gs_usb: add ethtool set_phys_id callback to locate physical device

This patch Implements the ethtool set_phys_id callback to ease the
locating of specific physical devices. Currently only supported on
candleLight interfaces.

Signed-off-by: Hubert Denkmair <hubert@denkmair.de>
Signed-off-by: Maximilian Schneider <max@schneidersoft.net>
[mkl: split codingstyle change sinto separate patch]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>