cpufreq: governor: Change confusing struct field and variable names

The name of the prev_cpu_wall field in struct cpu_dbs_info is
confusing, because it doesn't represent wall time, but the previous
update time as returned by get_cpu_idle_time() (that may be the
current value of jiffies_64 in some cases, for example).

Moreover, the names of some related variables in dbs_update() take
that confusion further.

Rename all of those things to make their names reflect the purpose
more accurately.  While at it, drop unnecessary parens from one of
the updated expressions.

No functional changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Chen Yu <yu.c.chen@intel.com>
diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
index 3e0eb7c..34eb214 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -111,7 +111,7 @@
 /* Per cpu structures */
 struct cpu_dbs_info {
 	u64 prev_cpu_idle;
-	u64 prev_cpu_wall;
+	u64 prev_update_time;
 	u64 prev_cpu_nice;
 	/*
 	 * Used to keep track of load in the previous interval. However, when