Staging: comedi: initialize divisor variables

The i8253_cascade_ns_to_timer_2div() function (and
i8253_cascade_ns_to_timer macro) checks the old values *d1 and *d2
for correctness as a heuristic before calculating new values.  Don't
call the function with uninitialized values in *d1 and *d2.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>