cpufreq: warn about invalid vals to scaling_max/min_freq interfaces

When echo an invalid val to scaling_min_freq:
> echo 123abc123 > scaling_min_freq
It looks weird to have a return val of 0:
> echo $?
> 0

Sane people won't echo strings like that into these interfaces but fuzz
tests may do. Also, maybe it's better to inform people if input is
invalid.

After this:
> echo 123abc123 > scaling_min_freq
> -bash: echo: write error: Invalid argument

Signed-off-by: qinyu <qinyu32@huawei.com>
Tested-by: zhangxiaofeng <zhangxiaofeng46@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>