PM / OPP: passing NULL to PTR_ERR()

The code was using PTR_ERR(NULL) which causes a static checker warning.
I have fixed up the printks and changed the return to -ENOENT.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>