net: Convert cfg802154_pernet_ops

These pernet_operations have only exit method, which
moves devices from cfg802154_rdev_list to init_net.
This may occur in any time from nl802154_wpan_phy_netns(),
so we are nice with rtnl_lock() synchronization.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>