The suspected BSE case reported yesterday on the Italian island of Sardinia, some 150 miles from the mainland, has  been confirmed positive in the first case of the disease beyond the mainland. The case involved a 4-year-old cow from a farm in the province of Oristano.   Until now, all cases had been confined to scattered parts of the peninsula, mainly in the north, with the major islands of Sardinia and Sicily spared from BSE, although both have livestock operations.