USB: twl4030-usb: Report correct vbus value for accessory charger adapters1

The twl4030-usb driver exports the status of VBUS as sysfs attribute.
In case an accessory charger adapter (ACA) is connected to the OTG
transceiver the attribute is always 'off', even when the charger
provides VBUS. Added a variable to keep track of the status of VBUS
and report it correctly

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@tomtom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>