staging: comedi: add 'pacer' member to struct comedi_device

Add a new member to the comedi_device struct for a comedi_8254 'pacer'. This
provides a convient place to store the data allocated by the comedi_8254 module
for boards that use an 8254 device to create the data acquisition pacer clock.

Automatically free this pointer in comedi_device_detach_cleanup() so that the
drivers don't need to do it when then are detached.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>