staging: comedi: drivers should not clear the async->events

The comedi core resets the async->events in comedi_buf_reset() which is
called when the subdevice is restored to an idle state and at the start
of an async command. The async->events are also cleared after handling
the events in comedi_event().

Drivers should not clear the events manually.

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>