usb: dwc3: pci: fix build warning on !PM_SLEEP

usb: dwc3: pci: fix build warning on !PM_SLEEP

When building a kernel with CONFIG_PM_SLEEP=n, we
get the following warning:

drivers/usb/dwc3/dwc3-pci.c:253:12: warning: 'dwc3_pci_pm_dummy' defined but not used

In order to fix this, we should only define
dwc3_pci_pm_dummy() when CONFIG_PM_SLEEP is defined.

Fixes: 0c6e7c32a847 ("usb: dwc3: pci: runtime_resume child device")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>