staging: comedi: comedi_fops: remove unnecessary s->async use

In places where the `s->async` value has been stored in a local
variable, use the variable instead of repeating `s->async`.

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>

diff --cc drivers/staging/comedi/comedi_fops.c
Simple merge