drivers/clock_control: stm32u5: Use LL API to set MSI as PLL srce
drivers/clock_control: stm32u5: Use LL API to set MSI as PLL srce

Recent changes in this function made the code now compatible
with the use of LL API to perform this configuration.
Use it to simplify the driver.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr