staging: comedi: remove 'settling_time_0' from subdevice

The `settling_time_0` member of `struct comedi_subdevice` can be set by
a low-level comedi driver and will be copied to user-space as part of
the information provided by the `COMEDI_SUBDINFO` ioctl.  No comedi
driver has ever set it; it's just been left at its initial value of 0.
Remove it to save a bit of space, and behave as though it is 0.

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