usb: dwc3: gadget: Move vbus draw to workqueue context

In the Linux kernel, the following vulnerability has been resolved:

usb: dwc3: gadget: Move vbus draw to workqueue context

Currently dwc3_gadget_vbus_draw() can be called from atomic

context, which in turn invokes power-supply-core APIs. And

some these PMIC APIs have operations that may sleep, leading

to kernel panic.

Fix this by moving the vbus_draw into a workqueue context.