mailbox: mtk-cmdq: Fix pm_runtime_get_sync() warning in mbox shutdown

In the Linux kernel, the following vulnerability has been resolved: mailbox: mtk-cmdq: Fix pm_runtime_get_sync() warning in mbox shutdown 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().