usb: dwc3: get rid of ->prepare()/->complete()

Using ->prepare()/->complete() to mask/unmask
IRQs is wrong at least for dwc3. We need to
make sure that by the end of ->resume(), IRQs
are working and ready to fire because a child
device may need working IRQs for its own ->resume()
method.

Signed-off-by: Felipe Balbi <balbi@ti.com>