staging: comedi: amplc_dio200: implement timer subdevice

Implement the timer subdevice for the new PCIe boards.  The subdevice was
previously marked as unused, but was reserved for this purpose.

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