Cleanup for calc_L0S_latency() and calc_L1_latency().

  - Separate exit latency and acceptable latency calculation.
  - Some minor cleanups.
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>