cpufreq: loongson2: Unregister platform_driver on failure

When cpufreq_register_driver() returns error, the cpufreq_init() returns
without unregister platform_driver, fix by add missing
platform_driver_unregister() when cpufreq_register_driver() failed.

Fixes: f8ede0f7 ("MIPS: Loongson 2F: Add CPU frequency scaling support")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>