twl4030_charger: distinguish between USB current and 'AC' current

The twl4030 charger has two current sources, 'USB' and 'AC'
(presumably "Accessory Charger" because it isn't Alternating Current).

If 'AC' is providing current, we should set the current limit
differently to when it isn't (and so USB is used).
So split 'cur' into 'usb_cur' and 'ac_cur' and use accordingly.

Now we must review the current setting on any interrupt or USB
event which might indicate that the charger-source has changed.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Sebastian Reichel <sre@kernel.org>