can: peak_usb: add support of ethtool set_phys_id()

This patch makes it possible to specifically flash the LED of a CAN
port of the CAN-USB interfaces of PEAK-System.

Link: https://lore.kernel.org/r/20210309122141.3276927-1-mkl@pengutronix.de
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
[mkl: use common prefix PCAN_ for defines]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>