uas: new function to cancel data urbs

Add uas_unlink_data_urbs function to cancel in-flight data urbs.
Moves existing code into a separate function.

[ v2: also drop the locking, just call usb_unlink_urb no matter what,
      which is safe because the usb core guarantees the completion
      callback is called only once ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>