cpufreq: interactive: Fix division by zero in round_to_nw_start()

* Timer rate was 0 in init, causing round_to_nw_start() to
divide by zero. Fix this by setting timer_rate to timer_rate
dependent on max_freq as we boot with max_freq.

* Fix some other potential divisions by zero caused by
integer overflow when converting usecs to jiffies.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>