staging: comedi: amplc_pci230: remove private data 'ai_scan_pos'

This member of the private data is replicating what the comedi_async
'cur_chan' member is used for. Use that instead and remove the private
data member.

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>