NEW YORK — R. Kelly has been placed on suicide watch at Brooklyn’s federal jail following his 30-year sentence for sex trafficking — and his lawyer says the disgraced R&B star has no business being there. The 55-year-old has been alone in a cell at the Metropolitan Detention Complex since his sentencing Wednesday for a 25-year scheme in which he sexually and mentally abused fans and other young women. Kelly’s lawyer Jennifer Bonjean told the Daily News he had prepared himself mentally for the steep prison term imposed by Brooklyn Federal Court Judge Ann Donnelly and was content in the general …