nrf_802154: nrf_802154_serialization_error never returns

Functions nrf_802154_serialization_error and
nrf_802154_sl_fault_handler shall never return.
However `__ASSERT()` might expand to an empty instruction
CONFIG_ASSERT=n and is not enough to stop execution.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>