Reset DMA channel correctly. (google#196)

- If we clear dmaInterrupt, the cs will be non-zero state
  (DreqStopsDMA|Paused|End) and IsAvaiable() returns false.
- nextCB is not supposed to be written directly.
  p.40, 4.2.1 DMA Channel Register Address Map
Only three registers in each channels register set are directly
writeable (CS, CONBLK_AD and DEBUG). The other registers
(TI, SOURCE_AD, DEST_AD, TXFR_LEN, STRIDE & NEXTCONBK),
are automatically loaded from a Control Block data structure held
in external memory.

host/bcm283x/dma.go

@@ -514,11 +514,7 @@ func (d *dmaChannel) isAvailable() bool {
 // It doesn't clear the local controlBlock cached values.
 func (d *dmaChannel) reset() {
 	d.cs = dmaReset
-	// Clear bits if needed.
-	// TODO(simokawa): Test if it works as expected.
-	d.cs = dmaInterrupt
 	d.cbAddr = 0
-	d.nextCB = 0
 }
 
 // startIO initializes the DMA channel to start a transmission.