cpufreq: loongson3: Check for error code from devm_mutex_init() call

Even if it's not critical, the avoidance of checking the error code
from devm_mutex_init() call today diminishes the point of using devm
variant of it. Tomorrow it may even leak something. Add the missed
check.

Fixes: ccf51454 ("cpufreq: Add Loongson-3 CPUFreq driver support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>