Implemented DMA controller disable, PC2086/3086 self test works properly.

src/dma.c

@@ -252,6 +252,9 @@ int dma_channel_read(int channel)
 {
         uint16_t temp;
 
+	if (dma.command & 0x04)
+		return DMA_NODATA;
+		
         if (!AT)
                 refreshread();
         
@@ -317,6 +320,9 @@ int dma_channel_read(int channel)
 
 int dma_channel_write(int channel, uint16_t val)
 {
+	if (dma.command & 0x04)
+		return DMA_NODATA;
+
         if (!AT)
                 refreshread();