natsemi: Use round_jiffies() for slow timers

Unless we have failed to fill the RX ring the timer used by the natsemi
driver is not particularly urgent and can use round_jiffies() to allow
grouping with other timers.

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