Oct 28 (Reuters) - Goldman Sachs raised Texas Instruments Inc to its highest rating on greater potential for margin expansion, while cutting flash memory maker SanDisk Corp by a notch to "neutral" on valuation. Near-term downside risk for SanDisk stock should be limited as long as NAND supply-demand is in balance and pricing is stable, analyst James Covello wrote in a note. NAND chips are used ...