PM / devfreq: Rewrite devfreq_update_status() to fix multiple bugs

The current devfreq_update_status() has the following bugs:
- If previous frequency doesn't have a valid level, it does an out of bounds
  access into the trans_table and causes memory corruption.
- When the new frequency doesn't have a valid level, the time spent in the
  new frequency is counted towards the next valid frequency switch instead of
  being ignored.
- The time spent on the previous frequency is added to the new frequency's
  stats instead of the previous frequency's stats.

This patch fixes all of this.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>