staging: comedi: rtd520: clear FIFO when canceling async command

Clear the A/D FIFO as part of the analog input (*cancel) to help with
cleaning up the async command.

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>