staging: comedi: das16: deschedule timer routine on detach

The "das16" driver optionally uses a kernel timer and a DMA channel to
support asynchronous data acquisition, but currently never calls
`del_timer_sync()`.  There is some possibility the timer routine could
still be scheduled to run when the comedi "detach" handler is run to
clean up the device and cause a certain amount of havoc.  Avoid that by
calling `del_time_sync()` in the comedi "detach" handler
`das16_detach()` if the timer was initialized by the "attach" handler
`das16_attach()`.  Use the timer's `data` member to tell whether it was
initialized or not.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>