staging: comedi: addi_apci_2032: support stop_src == TRIG_COUNT

When setting up asynchronous commands on the special interrupt
subdevice, support the `TRIG_COUNT` stop source to allow the command to
stop automatically after a specified number of scans.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>