cpufreq: Replace pointer subtraction with iteration macro

cpufreq: Replace pointer subtraction with iteration macro

The cpufreq documentation suggests avoiding direct pointer subtraction
when working with entries in driver_freq_table, as it is relatively
costly. Instead, the recommended approach is to use the provided
iteration macros, like cpufreq_for_each_valid_entry_idx().

Use cpufreq_for_each_entry_idx() instead of pointer subtraction in
cpufreq_frequency_table_cpuinfo() which improves code clarity and
follows the established cpufreq coding style.

While at it, remove redundant local variable initialization from
cpufreq_table_index_unsorted().

No functional change intended.

Signed-off-by: Zihuan Zhang <zhangzihuan@kylinos.cn>
Link: https://patch.msgid.link/20250923075553.45532-1-zhangzihuan@kylinos.cn
[ rjw: Subject tweak, changelog edits, local variable definition tweak ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>