staging: comedi: ni_at_ao: use 8253.h helpers

To better document the code, use the 8254.h helpers to set the
timers instead of doing the outb() instructions.

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>