mac802154: move dev_hold out of ieee802154_if_add

This patch moves the dev_hold call inside of nl-phy ieee802154_add_iface
function. The ieee802154_add_iface is the only one function which use the
ieee802154_if_add function and contains the corresponding dev_put call.

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