can: peak_usb: constify struct peak_usb_adapter

A "struct peak_usb_adapter" describes a certain USB adapter, as this doesn't
change during runtime, this patch marks all USB adapter definitions as const.

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