cpufreq: unify show() and store() naming and use __ATTR_XX

Usually, sysfs attributes have .show and .store and their naming
convention is filename_show() and filename_store().

But in cpufreq the naming convention of these functions is
show_filename() and store_filename() which prevents __ATTR_RW() and
__ATTR_RO() from being used in there to simplify code.

Accordingly, change the naming convention of the sysfs .show and
.store methods in cpufreq to follow the one expected by __ATTR_RW()
and __ATTR_RO() and use these macros in that code.

Signed-off-by: Lianjie Zhang <zhanglianjie@uniontech.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>