drivers: ieee802154_nrf5: skip Enh Ack conf for invalid short address

If the invalid short address (0xfffe) is specified,
configure the Enhanced Ack just for the extended address.
This is needed because Header IEs for Enhanced Ack must be
configured before the short address has been assigned to
the child.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>