can: peak_usb: add adapter BEC callback definition

Add the definition of a new callback that enable any PEAK-System CAN USB
adapter to grant read access to its Bus Error Counters value. This ability is
not supported by all the PEAK-System adapters, thus, for those, the callback
pointer will be initiaized to NULL, which is correct regarding the linux-can
device driver specs.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>