hwmon: (applesmc) Limit key length in warning messages

Key lookups may call read_smc() with a fixed-length key string,
and if the lookup fails, trailing stack content may appear in the
kernel log. Fixed with this patch.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>