cpufreq: cppc: Fix invalid return value in .get() callback

Returning a negative error code in a function with an unsigned
return type is a pretty bad idea. It is probably worse when the
justification for the change is "our static analisys tool found it".

Fixes: cf7de258 ("cppc_cpufreq: Fix possible null pointer dereference")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Lifeng Zheng <zhenglifeng1@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>