staging: comedi: addi_common.h: remove 'ui_AiScanLength' from private data

This member of the private data is a copy of the cmd->scan_end_arg. Use that
instead.

Use a local variable in apci3120_cyclic_ai() for the DMA 'scan_bytes', which
is the cmd->scan_end_arg * 2. Replace the open-coded '2' with sizeof(short).

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>