drivers: thermal: st: remove several sparse warnings

Simple patch to make symbols static. Symbols that are not
shared with other parts of the kernel can be made static.
This change also removes several sparse complains.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Ajit Pal Singh <ajitpal.singh@st.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>