staging: comedi: pcl818: reduce indent level in pcl818_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.

Also, move the setting of the 'irq_was_now_closed' to the only place
wher it actually gets left set to 1 and remove the goto.

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>