bq2415x_charger: Add support for offline and 100mA mode

* Renamed mode BQ2415X_MODE_NONE to BQ2415X_MODE_OFF because this mode
  turning chaging completly off

* Added new mode BQ2415X_MODE_NONE which enable charging with maximal
  current limit 100mA (this is minimal safe value for bq2415x chips)

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>