thermal: cpu_cooling: use snprintf instead of sprintf

Limit the amount of bytes written to dev_name by
secure writing with snprintf.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>