Use DMA_NUM_CHANNELS for index if allocation fails

teensy3/DMAChannel.cpp

 		if (++ch >= DMA_NUM_CHANNELS) {
 			__enable_irq();
 			TCD = (TCD_t *)0;
-			channel = 16;
+			channel = DMA_NUM_CHANNELS;
 			return; // no more channels available
 			// attempts to use this object will hardfault
 		}