USB: serial: quatech2: fix use-after-free in probe error path

The interface read URB is submitted in attach, but was only unlinked by
the driver at disconnect.

In case of a late probe error (e.g. due to failed minor allocation),
disconnect is never called and we would end up with active URBs for an
unbound interface. This in turn could lead to deallocated memory being
dereferenced in the completion callback.

Fixes: f7a33e60 ("USB: serial: add quatech2 usb to serial driver")
Cc: stable <stable@vger.kernel.org>	# v3.5: 40d04738
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>