staging: comedi: das1800: allow setting the external trigger polarity

The external pin TGIN can be used to start and/or stop the analog input
command. Currently the driver only supports negative edge polarity for
this signal.

Add support to allow the user to select positive edge polarity using
the CR_INVERT flag.

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