staging: comedi: adv_pci1710: ai (*cancel) should not enable software trigger

The (*cancel) operation should do just that. Remove the setting of the SW bit
which enables the software trigger.

For aesthetics, rename the function so it has namespace associated with the
driver and add a couple comments.

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>