PM / OPP: Staticize __dev_pm_opp_remove()

Its a local routine and need not be accessible outside of opp.c.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 86453b473b1f68c238a6901b26158b4ca3b369bc)
Signed-off-by: Alex Shi <alex.shi@linaro.org>