USB: legousbtower: fix potential NULL-deref on disconnect

USB: legousbtower: fix potential NULL-deref on disconnect

commit cd81e6fa8e033e7bcd59415b4a65672b4780030b upstream.

The driver is using its struct usb_device pointer as an inverted
disconnected flag, but was setting it to NULL before making sure all
completion handlers had run. This could lead to a NULL-pointer
dereference in a number of dev_dbg and dev_err statements in the
completion handlers which relies on said pointer.

Fix this by unconditionally stopping all I/O and preventing
resubmissions by poisoning the interrupt URBs at disconnect and using a
dedicated disconnected flag.

This also makes sure that all I/O has completed by the time the
disconnect callback returns.

Fixes: 9d974b2a06e3 ("USB: legousbtower.c: remove err() usage")
Fixes: fef526cae700 ("USB: legousbtower: remove custom debug macro")
Fixes: 4dae99638097 ("USB: legotower: remove custom debug macro and module parameter")
Cc: stable <stable@vger.kernel.org>     # 3.5
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190919083039.30898-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>