The Golden Globe-winning actor and star of ‘Selma’ is currently tearing up the National Theatre stage in ‘Coriolanus’. But it’s a rare trip back home for the British-Nigerian actor who made his name in ‘Spooks’ but was forced moved to America in 2007 because of his struggle to find roles. He talks to Annabel Nugent about a tremendous work ethic inherited from his parents, his fears for his adopted home country and why excellence is the best weapon against prejudice