USB: mos7840: remove invalid disconnect handling

Remove private zombie flag used to signal disconnect and to prevent
control urb from being submitted from interrupt urb completion handler.

The control urb will not be re-submitted as both the control urb and the
interrupt urb is killed on disconnect.

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