drivers: clock_control: remove inline attributes in stm32 clock drivers

Remove the inline attribute from stm32 clock drivers functions
especially for functions that are only referenced in the clock API
operation structure and therefore cannot be inline. As a generic
comment, today compiler are smart enough to optimize embedded software
without needing inline function attributes in the source files.

This change hopes that next stm32 clock driver will no more replicate
this bad habit when using existing source files as implementation
examples.

No functional change.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>