USB: yurex: fix NULL-derefs on disconnect

USB: yurex: fix NULL-derefs on disconnect

The driver was using its struct usb_interface pointer as an inverted
disconnected flag, but was setting it to NULL without making sure all
code paths that used it were done with it.

Before commit ef61eb43ada6 ("USB: yurex: Fix protection fault after
device removal") this included the interrupt-in completion handler, but
there are further accesses in dev_err and dev_dbg statements in
yurex_write() and the driver-data destructor (sic!).

Fix this by unconditionally stopping also the control URB at disconnect
and by using a dedicated disconnected flag.

Note that we need to take a reference to the struct usb_interface to
avoid a use-after-free in the destructor whenever the device was
disconnected while the character device was still open.

Fixes: aadd6472d904 ("USB: yurex.c: remove dbg() usage")
Fixes: 45714104b9e8 ("USB: yurex.c: remove err() usage")
Cc: stable <stable@vger.kernel.org>     # 3.5: ef61eb43ada6
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191009153848.8664-6-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>