MIPS: ftrace: Reword prepare_ftrace_return() comment block

Improve the comment block for prepare_ftrace_return().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org