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

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().

Fixes: 8afe816b0c99 ("mailbox: mtk-cmdq-mailbox: Implement Runtime PM with autosuspend")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>