can: kvaser_usb: Rename message/msg to command/cmd

Rename message to command and msg to cmd, where appropriate. To make the
code more readable and to better match Kvaser's Linux drivers.
Signed-off-by: NJimmy Assarsson <extja@kvaser.com>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>