natsemi: Use NATSEMI_TIMER_FREQ consistently

The natsemi driver has a define NATSEMI_TIMER_FREQ which looks like it
controls the normal frequency of the chip poll timer but in fact only
takes effect for the first run of the timer.  Adjust the value of the
define to match that used by the timer and use the define consistently.

Signed-off-by: Mark Brown <broonie@sirena.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>