cpufreq: simplify boolean parsing with kstrtobool in store function

Update the cpufreq store function to use kstrtobool for parsing boolean
values. This simplifies the code and improves readability by using a
standard kernel function for boolean string conversion.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Link: https://lore.kernel.org/r/d392eba3bad1231776124c321cef8c184ce1482d.1718988436.git.perry.yuan@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>