usb: phy: twl4030-usb: check if vbus is driven by twl itself

At least on pandora, STS_VBUS gets set even when VBUS is driven by twl
itself. Reporting VBUS in this case confuses OMAP musb glue and charger
driver, so check if OTG VBUS charge pump is on before reporting VBUS
event to avoid this problem.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>