drm/nouveau/debugfs: Fix check of pm_runtime_get_sync failure

[ Upstream commit 909e9c9c428376e2a43d178ed4b0a2d5ba9cb7d3 ]

pm_runtime_get_sync returns negative on failure.

Fixes: eaeb9010bb4b ("drm/nouveau/debugfs: Wake up GPU before doing any reclocking")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>