By Lucia Mutikani WASHINGTON (Reuters) – U.S. retail sales rose more than expected in December, boosted by an increase in motor vehicle and online purchases, keeping the economy on solid ground heading into the new year. The upbeat report from the Commerce Department on Wednesday, which prompted economists to upgrade their economic growth estimates for the fourth quarter, cast further doubt on financial market expectations that the Federal Reserve would start cutting interest rates in March. It followed news earlier this month of strong employment and wage gains in December as well as a pickup…