。
The reservation has been confirmed.
Literal
Reservation-of confirmation [topic-は] finished-[polite-ました].
すむ ('to finish, be done, come to an end') is a v5m verb with broader semantic reach than 終わる — often used idiomatically to mean 'the matter is settled' or 'the process is complete.' Here, 確認はすみました is a crisp way of saying 'confirmation is complete.'