opp: remove WARN when no valid OPPs remain

[ Upstream commit 335ffab3ef864539e814b9a2903b0ae420c1c067 ]

This WARN can be triggered per-core and the stack trace is not useful.
Replace it with plain dev_err(). Fix a comment while at it.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>