staging: comedi: amplc_pci230: 'ao_scan_count' is always 'stop_arg'

When the cmd->stop_src == TRIG_COUNT, the 'ao_scan_count' is the cmd->stop_arg.
For any other stop_src the 'ao_scan_count' is 0, which is also the cmd->stop_arg.

Simplify the 'ao_scan_count' initialization.

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>