charger-manager: Clean up for better readability

This patch includes below changes:

* Some code change for better readability.
* Current code in probe already ensures desc->charger_regulators
  is not NULL. No need to check if it is NULL or not before calling
  regulator_bulk_free().
* Use dev_get_drvdata() in cm_suspend_prepare() and cm_suspend_complete()

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>