usb: musb: omap2430: turn off vbus on cable disconnect

On USB_EVENT_ID event the musb glue enables VBUS by calling
omap2430_musb_set_vbus(musb, 1) that sets the session bit, but on
USB_EVENT_NONE reverse action is never made, and that breaks PM.

Disable VBUS on USB_EVENT_NONE to be sure musb session is ended
on cable unplug so that PM works.

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