cc253x: DMA Changes

- Fixed DMA irq flag clearing
- Added a dma_reset helper

See Pull Request #18

cpu/cc253x/dev/dma_intr.c

@@ -47,21 +47,21 @@ dma_isr(void) __interrupt (DMA_VECTOR)
   DMAIF = 0;
 #ifdef HAVE_RF_DMA
   if((DMAIRQ & 1) != 0) {
-    DMAIRQ &= ~1;
+    DMAIRQ = ~1;
     DMAARM=0x81;
     rf_dma_callback_isr();
   }
 #endif
 #ifdef SPI_DMA_RX
   if((DMAIRQ & 0x08) != 0) {
-    DMAIRQ &= ~(1 << 3);
+    DMAIRQ = ~(1 << 3);
     spi_rx_dma_callback();
   }
 #endif
 #if DMA_ON
   for(i = 0; i < DMA_CHANNEL_COUNT; i++) {
     if((DMAIRQ & (1 << i)) != 0) {
-      DMAIRQ &= ~(1 << i);
+      DMAIRQ = ~(1 << i);
       if(dma_callback[i] != 0) {
         process_poll(dma_callback[i]);
       }