The salacious advertisements being stripped off scores of city buses and subway platforms for their hidden sexual message also contain a secret reference to the N-word. Stamped faintly below the logo for the ads' sponsor, the hip-hop clothing line Akademiks, are the initials "H.N.I.C." - slang for "Head [N-----] In Charge." A subway operator spotted the offensive acronym and alerted the Daily News yesterday. "It's sad, it really is," said the L-train operator, who asked not to be identified. As The News first reported yesterday, Akademiks admitted to planting the slang for oral sex - "Get Brain" - into the...