usb: dwc3: Describe 'wakeup_work' field of struct dwc3_pci

Describe 'wakeup_work' field of struct dwc3_pci to avoid a warning:

drivers/usb/dwc3/dwc3-pci.c:59: warning: Function parameter or member 'wakeup_work' not described in 'dwc3_pci'

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>