[xhci] Use meaningful device names in debug messages

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/usb/xhci.h

@@ -1036,6 +1036,8 @@ struct xhci_pch {
 struct xhci_device {
 	/** Registers */
 	void *regs;
+	/** Name */
+	const char *name;
 
 	/** Capability registers */
 	void *cap;