staging: comedi: pcl816: reduce indent level in pcl816_ai_cancel()

If an async command is not running the (*cancel) function doesn't do
anything. Exit the function early if this is the case.

This allows reducing the indent level in the rest of the function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>