cpufreq: SPEAr: Fix incorrect variable type

'clk_round_rate' returns a negative error code upon failure. This
will never get detected by unsigned 'newfreq'. Make it signed.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>