cpufreq: speedstep: fix speedstep_detect_processor()'s return type

speedstep_detect_processor() is declared as returing an
'enum speedstep_processor' but use an 'int' in its definition.

Fix this by using 'enum speedstep_processor' in its definition too.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>