powerpc/ftrace: Remove mod_return_to_handler

mod_return_to_handler is the same as return_to_handler, except
it handles the change of the TOC (r2). Add this into
return_to_handler and remove mod_return_to_handler.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>