riscv: ftrace: correct the condition logic in function graph tracer

The condition should be logical NOT to assign the hook address to parent
address. Because the return value 0 of function_graph_enter upon
success.

Fixes: e949b6db (riscv/function_graph: Simplify with function_graph_enter())
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: stable@vger.kernel.org
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>