staging: comedi: dt2811: add async command support for AI subdevice

The interrupt support available on this board is pretty limited but its
simple enough to give basic async command support.

This allows reading a single channel continuously using either the internal
or an external clock to trigger each conversion. The command can also use
the external trigger input to start the command if the external clock is
being used for conversions.

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>