staging: comedi: hwdrv_apci3120: fix 'b_AiCyclicAcquisition' usage

This private data member is set when an analog input async command is
started by apci3120_cyclic_ai() and cleared when the command finishes
or is canceled.

Rename the CamelCase member to 'ai_running' and convert it to a
bit-field flag.

Remove the unnecessary clearing of the flag before or after calling
apci3120_cancel().

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>