staging: comedi: 8253.h: tidy up the i8253_cascade_ns_to_timer*() users

Introduce a couple defines for the common 8254 oscillator base values
used in the comedi drivers and remove the custom defines and open
coded values.

Change the i8253_cascade_ns_to_timer_2div() calls in the drivers to
the more generic i8253_cascade_ns_to_timer(). They are identical due
to the #define in the 8253.h header.

Remove the extra mask by TRIG_ROUND_MASK of the 'round_mode' parameter
to i8253_cascade_ns_to_timer(). That function already handles the mask.

Tidy up all the calls to i8253_cascade_ns_to_timer().

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