staging: comedi: comedi_test: fix timer lock-up

Commit 24051247 ("staging: comedi: comedi_test: use
comedi_handle_events()") resulted in the timer routine
`waveform_ai_interrupt()` calling `comedi_handle_events()` instead of
`comedi_events()`.  That had the advantage of automatically stopping the
acquisition on overflow/error/end-of-acquisition conditions (by calling
the comedi subdevice's "cancel" handler), but currently results in the
timer routine locking when one of those conditions occur.  This is
because the "cancel" handler `waveform_ai_cancel()` calls
`del_timer_sync()`.

Fix it by adding a bit to the device private data that indicates whether
the acquisition is active or not, and changing the "cancel" handler to
use `del_timer()` instead of `del_timer_sync()`.  The bit is set when
starting the acquisition, cleared when ending the acquisition (in the
"cancel" handler), and tested in the timer routine, which will do
nothing if the acquisition is inactive.  Also, make sure any scheduled
timeout event gets cancelled when the low-level device gets "detached"
from the comedi core by calling `del_timer_sync()` in the "detach"
handler `waveform_detach()`.

Fixes: 24051247 ("staging: comedi: comedi_test: use comedi_handle_events()")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>