cpufreq: schedutil: Use unsigned int for iowait boost

Make iowait_boost and iowait_boost_max as unsigned int since its unit
is kHz and this is consistent with struct cpufreq_policy.  Also change
the local variables in sugov_iowait_boost() to match this.

Signed-off-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>