staging: comedi: adv_pci1710: remove 'ai_do' from private data

This member of the private data is set to the "mode" that the ai command
is operating in but nothing uses it. Just remove it.

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>