In an effort to halt its growing deficit, Pacific Savings Bank in Costa Mesa sold its 50% stake in a data-processing subsidiary to Coast Savings & Loan, a Los Angeles-based S&L that already owned the other half of the subsidiary. Terms of the deal, including the sale price, were not disclosed. The subsidiary, Data Line Service in Covina, provides information on on-line checking, savings and loan accounts to about 125 S&Ls with combined assets of $50 billion in California, Nevada and Arizona. With a $50-million loss in the first nine months last year, Pacific Savings posted the fifth largest loss among Orange County's 34 S&Ls. At the end of September, the institution had $1.26 billion in assets but a negative net worth of $110 million.