doc: linux-wpan: Change the old function names to the lastest function names

The function declaration in the lastest include/net/mac802154.h has been
changed since v3.19.

ieee802154_alloc_device => ieee802154_alloc_hw
ieee802154_free_device => ieee802154_free_hw
ieee802154_register_device => ieee802154_register_hw
ieee802154_unregister_device => ieee802154_unregister_hw

However, the description in the Device drivers API section of
Documentation/networking/ieee802154.txt is still in the state of
v3.18.63.

Signed-off-by: Jian-Hong Pan <starnight@g.ncu.edu.tw>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>