urtwn: fix memory leak on device restart

Free data buffers every time when device is stopped, not when
it is detached; they are allocated at the initialization stage.

How-to-reproduce:

1. ifconfig wlan0 create wlandev urtwn0 up
2. vmstat -m | grep USBdev
3. service netif restart
4. vmstat -m | grep USBdev

Also, remove usbd_transfer_drain() call; it is already called by
usbd_transfer_unsetup().

Tested with RTL8188CUS, STA mode.

Approved by: re (marius)