Fix DMAChannel transferCount (KurtE)

teensy3/DMAChannel.h

 
 	// Set the number of transfers (number of triggers until complete)
 	void transferCount(unsigned int len) {
-		if (len > 32767) return;
-		if (len >= 512) {
+		if (!(TCD->BITER & DMA_TCD_BITER_ELINK)) {
+			if (len > 32767) return;
 			TCD->BITER = len;
 			TCD->CITER = len;
 		} else {
+			if (len > 511) return;
 			TCD->BITER = (TCD->BITER & 0xFE00) | len;
 			TCD->CITER = (TCD->CITER & 0xFE00) | len;
 		}