mac802154: remove const for non pointer in cfg ops

This patches removes the const keyword in variables which are non
pointers. There is no sense to declare call by value parameters as
const.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>