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