staging: comedi: pcl816: clarify 'irq_was_now_closed' flag in private data

This flag in the private data is set when an async command is canceled with
the ai (*cancel) operation. Rename the flag to 'ai_cmd_canceled' to clarify
its use.

Move the check for the flag in the interrupt handler. If the async command
was canceled there is no reason to handle the interrupt. Just clear interrupt
and return.

Also, make sure to clear the interrupt when the device is not attached.

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>