Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32F303/RCC/CFGR2#0x0
Clock configuration register 2
PREDIV division factor
ADC1 and ADC2 prescaler
ADC3 and ADC4 prescaler
https://github.com/cmsis-svd/cmsis-svd-data