staging: comedi: das1800: use comedi_timeout()

Use the helper function to handle the busywaiting for the analog
input conversion to complete.

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>