staging: comedi: comedi_fops.c: document exported functions

Add kerneldoc for exported functions `comedi_dev_put()`,
`comedi_dev_get_from_minor()`, `comedi_is_subdevice_running()`, and
`comedi_event()`.

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>