staging: comedi: 8253.h: rename i8253_cascade_ns_to_timer_2div()

All of the comedi drivers use the i8253_cascade_ns_to_timer macro
when calculating the divisors for the cascaded timers. This macro
just causes the i8253_cascade_ns_to_timer_2div() inline to be used.

Rename i8253_cascade_ns_to_timer_2div() and remove the macro.

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>