twl4030_charger: trust phy to determine when USB power is available.

The usb phy driver already determines when VBUS is available,
so repeating the test in the charger driver is pointless duplication.

On probe, process the last event from the phy, and from then on,
do whatever the phy tells us without double-checking.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Sebastian Reichel <sre@kernel.org>