PM / OPP: Drop unlikely before IS_ERR(_OR_NULL)

IS_ERR(_OR_NULL) already contain an 'unlikely' compiler flag and there
is no need to do that again from its callers. Drop it.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 50a3cb04a5f9cd5323a76db9ee409a7f3004259a)
Signed-off-by: Alex Shi <alex.shi@linaro.org>