musb_hdrc: Stop VBUS draw on usb_gadget_unregister_driver

If musb_gadget_vbus_draw() also toggles a clock like tusb does,
USB clock will stay on after rmmoding a gadget driver. This
can block omap idle states.

Also handle disconnect as b_idle so that musb_gadget_vbus_draw()
gets called.

Signed-off-by: Tony Lindgren <tony@atomide.com>