Last February, Plainclothes Patrolman Frank Serpico knocked at the door of a suspected Brooklyn heroin pusher. When the door opened a crack, Serpico shouldered his way in only to be met by a .22-cal. pistol slug crashing into his face. Somehow he survived, although there are still buzzing fragments in his head, causing dizziness and permanent deafness in his left ear. Almost as painful is the suspicion that he may well have been set up for the shooting by other policemen. For Serpico, 35, has been waging a lonely, four-year war against the routine and endemic corruption that he and others claim is rife in the New York City police department. His efforts are now sending shock waves through the ranks of New York's finest.. . . Though the impact of the commissions upcoming report has yet to be felt, Serpico has little hope that. . .