Mexico has granted permanent asylum in its embassy here to a man suspected of planting a bomb that exploded in a Chinese restaurant in August, slightly wounding six United States soldiers and a Honduran civilian. Charging that the Mexicans are harboring a ''terrorist'' in violation of all international conventions, the United States has lodged a ''firm'' behind-the-scenes protest with the Mexican Government in Mexico City, diplomats in Mexico said. But the Mexican Government has dismissed the American protest. Mexican diplomats called the man, Alfonso Guerrero Ulloa, 22 years old, a ''freedom fighter'' whose life was at risk because of his...