atusb: don't change csma settings while set channel

This patch fixes the behaviour to not overwrite csma settings when
set channel afterwards.

Cc: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>