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>

drivers/staging/comedi/comedi_fops.c

@@ -2035,7 +2035,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 
 	s = comedi_write_subdevice(dev, minor);
 	if (s && s->async) {
-		unsigned int bps = bytes_per_sample(s->async->subdevice);
+		unsigned int bps = bytes_per_sample(s);
 
 		poll_wait(file, &s->async->wait_head, wait);
 		comedi_buf_write_alloc(s->async, s->async->prealloc_bufsz);