ieee802154: 6lowpan: remove set to zero

Internal mechanism by calling netdev_alloc which use kzalloc already
sets these variables to zero. This patch cleanup the setup of net_device.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>