MIPS: Tracing: Cleanup the arguments passing of prepare_ftrace_return

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/1226/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>