mac802154: fix missing INIT_LIST_HEAD in ieee802154_if_add()

In the Linux kernel, the following vulnerability has been resolved:

mac802154: fix missing INIT_LIST_HEAD in ieee802154_if_add()

Kernel fault injection test reports null-ptr-deref as follows:

BUG: kernel NULL pointer dereference, address: 0000000000000008
RIP: 0010:cfg802154_netdev_notifier_call+0x120/0x310 include/linux/list.h:114
Call Trace:
<TASK>
raw_notifier_call_chain+0x6d/0xa0 kernel/notifier.c:87
call_netdevice_notifiers_info+0x6e/0xc0 net/core/dev.c:1944
unregister_netdevice_many_notify+0x60d/0xcb0 net/core/dev.c:1982
unregister_netdevice_queue+0x154/0x1a0 net/core/dev.c:10879
register_netdevice+0x9a8/0xb90 net/core/dev.c:10083
ieee802154_if_add+0x6ed/0x7e0 net/mac802154/iface.c:659
ieee802154_register_hw+0x29c/0x330 net/mac802154/main.c:229
mcr20a_probe+0xaaa/0xcb1 drivers/net/ieee802154/mcr20a.c:1316

ieee802154_if_add() allocates wpan_dev as netdev's private data, but not
init the list in struct wpan_dev. cfg802154_netdev_notifier_call() manage
the list when device register/unregister, and may lead to null-ptr-deref.

Use INIT_LIST_HEAD() on it to initialize it correctly.