USB: xhci: Fix compile error when CONFIG_PM=n

Fix this error when CONFIG_PM is not enabled:

drivers/usb/host/xhci.c:675: error: implicit declaration of function 'usb_root_hub_lost_power'

Wrap xhci_suspend() and xhci_resume() into an ifdef CONFIG_PM, along with
the functions that only they call -- xhci_save_registers() and
xhci_restore_registers().

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci.c

@@ -551,6 +551,7 @@ void xhci_shutdown(struct usb_hcd *hcd)
 		    xhci_readl(xhci, &xhci->op_regs->status));
 }
 
+#ifdef CONFIG_PM
 static void xhci_save_registers(struct xhci_hcd *xhci)
 {
 	xhci->s3.command = xhci_readl(xhci, &xhci->op_regs->command);
@@ -761,6 +762,13 @@ int xhci_resume(struct xhci_hcd *xhci, bool hibernated)
 	return 0;
 }
 
+#else
+
+#define	xhci_suspend	NULL
+#define	xhci_resume	NULL
+
+#endif	/* CONFIG_PM */
+
 /*-------------------------------------------------------------------------*/
 
 /**