twl4030_charger: add ac/mode to match usb/mode

This allows AC charging to be turned off, much like usb charging.
"continuous" mode is not available though.

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