staging: comedi: rtd520: use comedi_handle_events()

Use comedi_handle_events() to automatically (*cancel) the async command
for an end-of-acquisition or if an error/overflow occurs.

The code to cancel the async command can then be removed from rtd_interrupt().

Since the (*cancel) will also clear the FIFO, the ai_read_dregs() function
can also be removed.

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>