drivers: clock_control: stm32u5: Fix frequency calculation overflow
drivers: clock_control: stm32u5: Fix frequency calculation overflow

STM32u5 pllout frequency calculation overflows. Fix by changing
order of operations

Signed-off-by: Benjamin Bigler <e2ef60cbfe2dfa3b565b5731341d2fb622d6dd05@securiton.ch>

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