drivers/clock_control: stm32f1: Add explicit case for PLL source HSI

Now that we have specific bindings for STM32F100 devices,
we need dedicated treatment for PLL source HSI case.
Otherwise, we end up using undefined symbol STM32_PLL_PREDIV1.

Please note that previous code compiled, it was assigning
a wrong value to prediv. This had no consequence because
prediv value is forced in Cube LL functions.

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