mac802154: fix ieee802154_register_hw error handling

Currently if ieee802154_if_add failed, we don't unregister the wpan phy
which was registered before. This patch adds a correct error handling
for unregister the wpan phy when ieee802154_if_add failed.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>