isp1704_charger: Detect HUB/Host chargers

To avoid breaking high speed chirp handshaking with CDP
chargers, no more then 500mA should be drawn. To make sure
of this, utilizing current_max property. After the device
has enumerated, it's safe to draw the maximum 1800mA as
defined in the Battery Charging Specification. This can be
also used with normal USB connection if the controller sends
ENUMERATED notification with the milliamps as data.

From now on the online property indicates VBUS, present
property if there is a charger and current_max the milliamps
possible to draw from VBUS.

Signed-off-by: Heikki Krogerus <heikki.krogerus@nokia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>