PM / OPP: Remove useless check

Regulators are optional for devices using OPPs and the OPP core
shouldn't be printing any errors for such missing regulators.

It was fine before the commit 0c717d0f, but that failed to update
this part of the code to remove an 'always true' check and an extra
unwanted print message.

Fix that now.

Fixes: 0c717d0f (PM / OPP: Initialize regulator pointer to an error value)
Reported-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>