MIPS: Tracing: Cleanup of function graph tracer

Cleans up comments and ftrace_get_parent_addr() of function graph tracer.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips <linux-mips@linux-mips.org>
Cc: David Daney <david.s.daney@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/1231/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>