thermal: cpu_cooling: remove compiler warning

level will be used only if GET_FREQ mode is requested.
There is no potential harm with current code. But for
cleaning the compilation log, this patch initializes
level to zero.

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>