cpufreq: intel_pstate: Fix up iowait_boost computation

After commit b8bd1581 ("cpufreq: intel_pstate: Rework iowait
boosting to be less aggressive") the handling of the case when
the SCHED_CPUFREQ_IOWAIT flag is set again after a few iterations of
intel_pstate_update_util() is a bit inconsistent, because the
new value of cpu->iowait_boost may be lower than ONE_EIGHTH_FP
if it was set before, but has not dropped down to zero just yet.

Fix that up by ensuring that the new value of cpu->iowait_boost
will always be at least ONE_EIGHTH_FP then.

Fixes: b8bd1581 ("cpufreq: intel_pstate: Rework iowait boosting to be less aggressive")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>