[pxe] Report SUPPORTED_IRQ only if device supports interrupts

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

src/arch/i386/interface/pxe/pxe_undi.c

 	undi_get_iface_info->ServiceFlags =
 		( SUPPORTED_BROADCAST | SUPPORTED_MULTICAST |
 		  SUPPORTED_SET_STATION_ADDRESS | SUPPORTED_RESET |
-		  SUPPORTED_OPEN_CLOSE | SUPPORTED_IRQ );
+		  SUPPORTED_OPEN_CLOSE );
+	if ( netdev_irq_supported ( pxe_netdev ) )
+		undi_get_iface_info->ServiceFlags |= SUPPORTED_IRQ;
 	memset ( undi_get_iface_info->Reserved, 0,
 		 sizeof(undi_get_iface_info->Reserved) );