clock_control: stm32: code optimization

This commit brings code size optimization as suggested by Christer
Weinigel review during review of initial commit for this driver.
It also cleans up useless definition in header file.

Change-Id: Ibeaa2e51570dff21825c60c30ba83d939b31d938
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>