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>

drivers/usb/host/xhci-dwc3.c

@@ -111,6 +111,7 @@ void dwc3_set_fladj(struct dwc3 *dwc3_reg, u32 val)
 			GFLADJ_30MHZ(val));
 }
 
+#ifdef CONFIG_DM_USB
 static int xhci_dwc3_probe(struct udevice *dev)
 {
 	struct xhci_dwc3_platdata *plat = dev_get_platdata(dev);
@@ -184,3 +185,4 @@ U_BOOT_DRIVER(xhci_dwc3) = {
 	.platdata_auto_alloc_size = sizeof(struct xhci_dwc3_platdata),
 	.flags = DM_FLAG_ALLOC_PRIV_DMA,
 };
+#endif