staging: comedi: pcl812: use 8253.h helpers

Use the helper functions in 8253.h to clarify the timer programming.

Move start_pacer() to remove the need for the forward declarations.
Rename the function so it has namespace associated with the driver.

Change the 'mode' parameter. This parameter is really a flag to the
function indicating if the divisors should be loaded into the timers.

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>