[intel] Avoid spurious compiler warning on GCC 10

GCC 10 produces a spurious warning about an out-of-bounds array access
for the unsized raw dword array in union intelvf_msg.

Avoid the warning by embedding the zero-length array within a struct.

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

src/drivers/net/intelvf.c

@@ -52,14 +52,15 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  */
 static void intelvf_mbox_write ( struct intel_nic *intel,
 				 const union intelvf_msg *msg ) {
+	const struct intelvf_msg_raw *raw = &msg->raw;
 	unsigned int i;
 
 	/* Write message */
 	DBGC2 ( intel, "INTEL %p sending message", intel );
-	for ( i = 0 ; i < ( sizeof ( *msg ) / sizeof ( msg->dword[0] ) ) ; i++){
-		DBGC2 ( intel, "%c%08x", ( i ? ':' : ' ' ), msg->dword[i] );
-		writel ( msg->dword[i], ( intel->regs + intel->mbox.mem +
-					  ( i * sizeof ( msg->dword[0] ) ) ) );
+	for ( i = 0 ; i < ( sizeof ( *msg ) / sizeof ( raw->dword[0] ) ) ; i++){
+		DBGC2 ( intel, "%c%08x", ( i ? ':' : ' ' ), raw->dword[i] );
+		writel ( raw->dword[i], ( intel->regs + intel->mbox.mem +
+					  ( i * sizeof ( raw->dword[0] ) ) ) );
 	}
 	DBGC2 ( intel, "\n" );
 }
@@ -72,14 +73,15 @@ static void intelvf_mbox_write ( struct intel_nic *intel,
  */
 static void intelvf_mbox_read ( struct intel_nic *intel,
 				union intelvf_msg *msg ) {
+	struct intelvf_msg_raw *raw = &msg->raw;
 	unsigned int i;
 
 	/* Read message */
 	DBGC2 ( intel, "INTEL %p received message", intel );
-	for ( i = 0 ; i < ( sizeof ( *msg ) / sizeof ( msg->dword[0] ) ) ; i++){
-		msg->dword[i] = readl ( intel->regs + intel->mbox.mem +
-					( i * sizeof ( msg->dword[0] ) ) );
-		DBGC2 ( intel, "%c%08x", ( i ? ':' : ' ' ), msg->dword[i] );
+	for ( i = 0 ; i < ( sizeof ( *msg ) / sizeof ( raw->dword[0] ) ) ; i++){
+		raw->dword[i] = readl ( intel->regs + intel->mbox.mem +
+					( i * sizeof ( raw->dword[0] ) ) );
+		DBGC2 ( intel, "%c%08x", ( i ? ':' : ' ' ), raw->dword[i] );
 	}
 	DBGC2 ( intel, "\n" );
 }

src/drivers/net/intelvf.h

@@ -119,6 +119,12 @@ struct intelvf_msg_queues {
 	uint32_t dflt;
 } __attribute__ (( packed ));
 
+/** Raw mailbox message */
+struct intelvf_msg_raw {
+	/** Raw dwords */
+	uint32_t dword[0];
+} __attribute__ (( packed ));
+
 /** Mailbox message */
 union intelvf_msg {
 	/** Message header */
@@ -132,7 +138,7 @@ union intelvf_msg {
 	/** Queue configuration message */
 	struct intelvf_msg_queues queues;
 	/** Raw dwords */
-	uint32_t dword[0];
+	struct intelvf_msg_raw raw;
 };
 
 /** Maximum time to wait for mailbox message