Maybe fix for packets exact multiple of max sizes

Issue where multiple transfer structures were allocated, where  the transfer size is exact multiple of max size 16K, it allocated one extra before

ehci.cpp

 	transfer = allocate_Transfer();
 	if (!transfer) return false;
 	data = transfer;
-	for (count=(len >> 14); count; count--) {
-		next = allocate_Transfer();
-		if (!next) {
-			// free already-allocated qTDs
-			while (1) {
-				next = (Transfer_t *)transfer->qtd.next;
-				free_Transfer(transfer);
-				if (transfer == data) break;
-				transfer = next;
+	if (len) {
+		for (count=((len-1) >> 14); count; count--) {
+			next = allocate_Transfer();
+			if (!next) {
+				// free already-allocated qTDs
+				while (1) {
+					next = (Transfer_t *)transfer->qtd.next;
+					free_Transfer(transfer);
+					if (transfer == data) break;
+					transfer = next;
+				}
+				return false;
 			}
-			return false;
+			data->qtd.next = (uint32_t)next;
+			data = next;
 		}
-		data->qtd.next = (uint32_t)next;
-		data = next;
-	}
+	}	
 	// last qTD needs info for followup
 	data->qtd.next = 1;
 	data->pipe = pipe;