hwmon: (applesmc) Add temperature sensor labels to sysfs interface

The Apple SMC uses a systematic labeling scheme for the hardware
temperature sensors. This scheme is currently hidden from
userland. Since the sensor set, and consequently the numbering,
differs between models, an extensive database of configurations is
required for an application such as fan control. This patch adds the
SMC labels to the hwmon sysfs interface, allowing applications to use
the sensors more intelligibly.

[rydberg@euromail.se: fixed error handling]
Signed-off-by: Alex Murray <murray.alex@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Jean Delvare <khali@linux-fr.org>