PM / OPP: Don't WARN on multiple calls to dev_pm_opp_set_regulators()

If a platform specific OPP driver has called this routine first and set
the regulators, then the second call from cpufreq-dt driver will hit the
WARN_ON(). Remove the WARN_ON(), but continue to return error in such
cases.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>