Introduce enum coretemp_attr_index to better describe the index of each
sensor attribute.

No functional change.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20240202092144.71180-5-rui.zhang@intel.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>