staging: comedi: dmm32at: make AI (*cancel) actually cancel async command

Currently the AI (*cancel) changes a private data member to cause the
interrupt handler to cancel the async command after the next interrupt.

Move the code that disables the interrupt and terminates the acquisition
from the interrunt handler into dmm32at_ai_cancel() so that the async
command is terminated instantly.

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>