can: kvaser_usb: make use of units.h in assignment of frequency

Use the MEGA define plus the comment /* Hz */ when assigning
frequencies.

Link: https://lore.kernel.org/all/20211210075803.343841-1-mkl@pengutronix.de


Signed-off-by: Jimmy Assarsson <extja@kvaser.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>