cpufreq: persistent_stats: Synchronize access to time_in_state

The cpufreq_stats_create_table function does not fully protect cpu_stats'
members with a spinlock. This change fixes that. It also performs a
null-check on freq_table in freq_table_get_index and returns -ENOENT
appropriately.

Change-Id: I301d1b23ef766a77161bab1472dc810d53b6c5fe
Signed-off-by: Anurag Singh <anursing@codeaurora.org>