cpufreq: schedutil: Examine the correct CPU when we update util

After commit 674e7541 (sched: cpufreq: Allow remote cpufreq
callbacks) we stopped to always read the utilization for the CPU we
are running the governor on, and instead we read it for the CPU
which we've been told has updated utilization.  This is stored in
sugov_cpu->cpu.

The value is set in sugov_register() but we clear it in sugov_start()
which leads to always looking at the utilization of CPU0 instead of
the correct one.

Fix this by consolidating the initialization code into sugov_start().

Fixes: 674e7541 (sched: cpufreq: Allow remote cpufreq callbacks)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Reviewed-by: Patrick Bellasi <patrick.bellasi@arm.com>
Reviewed-by: Brendan Jackman <brendan.jackman@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>