tty: Reorder proc_set_tty() and related fns

Move the controlling tty-related functions and remove forward
declarations for __proc_set_tty() and proc_set_tty().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>