USB: quatech2: fix memory leak in error path

Fix memory leak in attach error path where the read urb was never freed.

Cc: Bill Pemberton <wfp5p@virginia.edu>
Cc: <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>