riscv: Fix fill_callchain return value

perf_callchain_store return 0 on success, -1 otherwise,
fix fill_callchain to return correct bool value.

Fixes: dbeb90b0

 ("riscv: Add perf callchain support")
Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>