isp1704_charger: Fix driver to work with changes introduced in v3.5

* omap musb driver does not report USB_EVENT_ENUMERATED event anymore
* omap musb driver reporting USB_EVENT_VBUS when charger is connected
* read last event from phy->last_event (instead from ulpi register)
* do not call wall charger detection more times

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