Is the trigger for Raging Soul a permanent effect or does it only last for the turn that a card is discarded?

The effect granted to Raging Soul when you discard a card lasts only for the current turn.

According to the RiftJudge FAQ (ID 2777 and 5001) regarding Raging Soul:

• This is a passive effect that constantly checks if you have discarded a card on the current turn.
• If you discard a card, Raging Soul gains Assault and Ganking until that turn ends (including through the end of turn phase).
• The ability turns off when the next turn starts, meaning you must discard a card again on a future turn to reactivate the ability.