cpufreq: ondemand: Use cpumask_var_t for on-stack cpu mask

A cpumask structure on the stack can cause a warning with
CONFIG_NR_CPUS=8192 (e.g. Ubuntu 22.04 uses this):

drivers/cpufreq/cpufreq_ondemand.c: In function 'od_set_powersave_bias':
drivers/cpufreq/cpufreq_ondemand.c:449:1: warning: the frame size of
	1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]
  449 | }
      | ^

CONFIG_CPUMASK_OFFSTACK=y is enabled by default for most distros, and
hence we can work around the warning by using cpumask_var_t.

Signed-off-by: Zhao Liu <zhao1.liu@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>