PM / OPP: Initialize regulator pointer to an error value

We are currently required to do two checks for regulator pointer:
IS_ERR() and IS_NULL().

And multiple instances are reported, about both of these not being used
consistently and so resulting in crashes.

Fix that by initializing regulator pointer with an error value and
checking it only against an error.

This makes code more consistent and more efficient.

Fixes: 7d34d56e

 (PM / OPP: Disable OPPs that aren't supported by the regulator)
Reported-and-tested-by: Jon Hunter <jonathanh@nvidia.com>
Reported-and-tested-by: Tony Lindgren <tony@atomide.com>
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Initialize to -ENXIO ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>