staging: comedi: remove redundant pointer dereference in comedi_poll()

`s->async->subdevice` in `comedi_poll()` points to the same `struct
comedi_subdevice` as `s`, so the double pointer reference is redundant.
Just use `s`.

(`s->async->subdevice` is initialized by
`__comedi_device_postconfig_async()` in
"drivers/staging/comedi/drivers.c" and doesn't change.)

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>