xen/sound: switch xen_snd_front to use INVALID_GRANT_REF

Instead of using a private macro for an invalid grant reference use
the common one.

Signed-off-by: Juergen Gross <[e-mail]>
Reviewed-by: Oleksandr Tyshchenko <[e-mail]>
Tested-by: Oleksandr Tyshchenko <[e-mail]> # Arm64 only
Signed-off-by: Juergen Gross <[e-mail]>

sound/xen/xen_snd_front_evtchnl.c

···
 		xenbus_free_evtchn(front_info->xb_dev, channel->port);
 
 	/* End access and free the page. */
-	if (channel->gref != GRANT_INVALID_REF)
+	if (channel->gref != INVALID_GRANT_REF)
 		gnttab_end_foreign_access(channel->gref, page);
 	else
 		free_page(page);
···
 	channel->index = index;
 	channel->front_info = front_info;
 	channel->state = EVTCHNL_STATE_DISCONNECTED;
-	channel->gref = GRANT_INVALID_REF;
+	channel->gref = INVALID_GRANT_REF;
 	page = get_zeroed_page(GFP_KERNEL);
 	if (!page) {
 		ret = -ENOMEM;

sound/xen/xen_snd_front_evtchnl.h

···
 
 struct xen_snd_front_info;
 
-#ifndef GRANT_INVALID_REF
-/*
- * FIXME: usage of grant reference 0 as invalid grant reference:
- * grant reference 0 is valid, but never exposed to a PV driver,
- * because of the fact it is already in use/reserved by the PV console.
- */
-#define GRANT_INVALID_REF	0
-#endif
-
 /* Timeout in ms to wait for backend to respond. */
 #define VSND_WAIT_BACK_MS	3000