Rename Transfer to usbTransfer, it's internal to gousb.

Move usbTransfer and it's methods to transfer.go, it's not only used by
iso transfers.

usb/transfer.go

@@ -0,0 +1,63 @@
+// Copyright 2016 the gousb Authors.  All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package usb
+
+/*
+#include <libusb-1.0/libusb.h>
+
+int extract_data(struct libusb_transfer *xfer, void *data, int max, unsigned char *status);
+int submit(struct libusb_transfer *xfer);
+*/
+import "C"
+
+import (
+	"fmt"
+	"time"
+	"unsafe"
+)
+
+type usbTransfer struct {
+	xfer *C.struct_libusb_transfer
+	pkts []*C.struct_libusb_packet_descriptor
+	done chan struct{}
+	buf  []byte
+}
+
+func (t *usbTransfer) Submit(timeout time.Duration) error {
+	t.xfer.timeout = C.uint(timeout / time.Millisecond)
+	if errno := C.submit(t.xfer); errno < 0 {
+		return usbError(errno)
+	}
+	return nil
+}
+
+func (t *usbTransfer) Wait(b []byte) (n int, err error) {
+	select {
+	case <-time.After(10 * time.Second):
+		return 0, fmt.Errorf("wait timed out after 10s")
+	case <-t.done:
+	}
+	var status uint8
+	n = int(C.extract_data(t.xfer, unsafe.Pointer(&b[0]), C.int(len(b)), (*C.uchar)(unsafe.Pointer(&status))))
+	if status != 0 {
+		err = TransferStatus(status)
+	}
+	return n, err
+}
+
+func (t *usbTransfer) Close() error {
+	C.libusb_free_transfer(t.xfer)
+	return nil
+}