hwmon: (acpi_power_meter) replace open-coded kmemdup_nul

hwmon: (acpi_power_meter) replace open-coded kmemdup_nul

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

Let's refactor this kcalloc() + strncpy() into a kmemdup_nul() which has
more obvious behavior and is less error prone.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230926-strncpy-drivers-hwmon-acpi_power_meter-c-v5-1-3fc31a9daf99@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>