OPP: rate-limit debug messages when no change in OPP is required

Otherwise, when enabling a debug build and dynamic debug in the kernel, it
quickly floods the kernel ring buffer and makes debugging of other
subsystems almost impossible, unless manually disabled.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>