clock_control: stm32: fix a PLL init corner case

Commit 58c8d151 ("clock control:stm32: provide STM32Cube LL based
driver") added PLL initialization which disables the PLL before
configuring it.

This works if the system clock source is not the PLL (which is the
usual case for Zephyr booting out of chip reset). However, if the
SYSCLK source *is* the PLL, this halts execution.

To support this use case, switch the SYSCLK source to the
always-present HSI before disabling the PLL.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>