thermal: Fix for setting the thermal zone mode to enable/disable

Basically without this patch changing the mode of thermal zone
is not possible as wrong string size is passed to strncmp.

Signed-off-by: Amit Daniel Kachhap <amit.kachhap@linaro.org>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Len Brown <len.brown@intel.com>