The return value of pm_runtime_get_sync() in cmdq_mbox_shutdown()
will return 1 when pm runtime state is active, and we don't want to
get the warning message in this case.
So we change the return value < 0 for WARN_ON().
Fixes: 8afe816b0c99 ("mailbox: mtk-cmdq-mailbox: Implement Runtime PM with autosuspend")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@xxxxxxxxxxxx>