LONDON (Reuters) -British Prime Minister Boris Johnson suffered a further blow to his authority on Wednesday when his second ethics adviser in less than two years quit his post. Johnson last week survived a confidence ballot that saw 41% of his lawmakers vote against his leadership after months of scandals and gaffes that have raised questions over his authority to govern and knocked his standing with the public. Christopher Geidt, the independent adviser on ministers’ interests, last month said Johnson must explain why he thought he had not broken the ministerial code after being fined over a…