staging: comedi: remove async parameter from resize_async_buffer()

The `struct comedi_async *async` parameter of `resize_async_buffer()` is
redundant as its value can be easily derived from the `struct
comedi_subdevice *s` parameter as `s->async`.  Remove the parameter.

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>