hwmon: (applesmc) Avoid buffer overruns

gcc 7.1 complains that the driver uses sprintf() and thus does not validate
the length of output buffers.

drivers/hwmon/applesmc.c: In function 'applesmc_show_fan_position':
drivers/hwmon/applesmc.c:82:21: warning:
	'%d' directive writing between 1 and 5 bytes into a region of size 4

Fix the problem by using scnprintf() instead of sprintf() throughout the
driver. Also explicitly limit the number of supported fans to avoid actual
buffer overruns and thus invalid keys.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>