staging: comedi: fix oops for USB DAQ devices.

This fixes kernel oops when an USB DAQ device is plugged out while it's
communicating with the userspace software.

Signed-off-by: Bernd Porr <berndporr@f2s.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>