cpufreq: schedutil: Avoid WALT logic when WALT sysctl flag not set

When sysctl_sched_use_walt_cpu_util is set to 0, WALT stats are no longer
updated.  As a result, the schedutil governor throws warnings which lead to
a crash. Fix this by not running any WALT-related logic when
sysctl_sched_use_walt_cpu_util is 0.

Change-Id: Ibdbc5243c6f3d11071f8ab715ca76e0e840f8090
CRs-Fixed: 2089058
Signed-off-by: Jonathan Avila <avilaj@codeaurora.org>