blackfin-cpufreq: Change return type of cpu_set_cclk() to that of clk_set_rate()

The return type "unsigned long" was used by the cpu_set_cclk() function
while the type "int" is provided by the clk_set_rate() function.
Let us make this usage consistent.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>