staging: comedi: comedi_test: simplify time since last AI scan
The private data structure `struct waveform_private` currently uses
member `last` to remember the time of the last timer interrupt, and the
member `usec_remainder` to keep track of how far into a simulated scan
the interrupt occurred. Replace these with a single new member
`ai_last_scan_time` that records the time of the last scan. This
simplifies the calculation of the number of scans to simulate in the
timer routine, `waveform_ai_interrupt()`.
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>