ALSA: usb: fix usb build error when PM is not enabled

Fix build errors when CONFIG_PM is not enabled:

sound/usb/card.c:629: error: 'usb_audio_suspend' undeclared here (not in a function)
sound/usb/card.c:630: error: 'usb_audio_resume' undeclared here (not in a function)
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -606,6 +606,9 @@ static int usb_audio_resume(struct usb_interface *intf)
 
 	return 0;
 }
+#else
+#define usb_audio_suspend	NULL
+#define usb_audio_resume	NULL
 #endif		/* CONFIG_PM */
 
 static struct usb_device_id usb_audio_ids [] = {