While the stock isn't in the bargain bin, a return to net interest income growth should help it add onto its recent gains.Shares of Bank of America (BAC -2.21%) jumped Tuesday after the bank reported mixed second-quarter results, but indicated that it expects its net interest income to start growing again soon. Net interest income is the difference between the interest a bank earns from its loans and the interest it pays out on its deposits, and is an important metric for banks.Mixed resultsWhen Bank of Ame ...