staging: comedi: comedi_test: rename members for AI commands

Rename the members of `struct waveform_private` that are used to handle
AI commands, apart from those members used to control fake waveform
generation.  The renames are `timer` --> `ai_timer`, `scan_period` -->
`ai_scan_period`, and `convert_period` --> `ai_convert_period`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>