dm: usb: host: xhci-dwc3: add missing #ifdef CONFIG_DM_USB

dm: usb: host: xhci-dwc3: add missing #ifdef CONFIG_DM_USB

Add CONFIG_DM_USB flag to avoid following compilation errors
detected by buildman :
+drivers/usb/host/built-in.o: In function `xhci_dwc3_remove':
+drivers/usb/host/xhci-dwc3.c:168: undefined reference to `xhci_deregister'
+drivers/usb/host/built-in.o: In function `xhci_dwc3_probe':
+drivers/usb/host/xhci-dwc3.c:145: undefined reference to `usb_get_dr_mode'
+drivers/usb/host/xhci-dwc3.c:152: undefined reference to `xhci_register'

introduced by patch d5c3f014da3 "usb: host: xhci-dwc3: Convert driver to DM"

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reported-by: Ran Wang <ran.wang_1@nxp.com>