staging: comedi: adv_pci1710: tidy up start_pacer()

For aesthetics, rename this function so it has namespace associated
with the driver.

Change the parameters to the function. The 'mode' is really a flag to
load the counters and the divisors can be found in the private data.

To clarify the code and remove the magic numbers, use the 8253.h
helpers to set the timer mode and load the counters.

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>