President Reagan has wisely acted swiftly to allay congressional concern over the departure of Admiral Bobby R. Inman, whose professionalism was highly respected, as deputy director of central intelligence. But his task has not ended with the naming of another thoroughgoing professional, John N. McMahon, the CIA's current No. 3 man, to replace the admiral. For the buck does stop with the White House in preventing the kind of covert operations that have previously discredited the CIA - and in controlling the domestic intelligence operations that were reportedly opposed by Admiral Inman and Mr. McMahon. Senate confirmation hearings ought to bring assurances that Mr. McMahon will do his part to keep intelligence legitimate and effective.