staging: comedi: comedi_fops: always clear events

`comedi_event()` is called from low-level drivers to handle asynchronous
command event flags that are stored in `s->async->events` for subdevice
`s`.  It normally clears the event flags as well.  As a safety check, it
does nothing if no asynchronous command is running, but it leaves
`s->async->events` unchanged in this case.  For additional safety,
change it to always clear the event flags to avoid leaving stale event
flags set when another asynchronous command is set up.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>