staging: comedi: usbduxsigma: kill all urbs before freeing them

For aesthetic reasons, kill all the urbs before freeing them.
Also, remove a redundant killing of the ai and ao urbs in the
(*detach).

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>