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

The interface read and event URBs are submitted in attach, but were
never explicitly unlinked by the driver. Instead the URBs would have
been killed by usb-serial core on disconnect.

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

Fixes: ee467a1f ("USB: serial: add Moxa UPORT 12XX/14XX/16XX
driver")
Cc: stable <stable@vger.kernel.org>	# v3.14
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>