Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32f3/STM32F302/RCC/CFGR2#0x0
Clock configuration register 2
PREDIV division factor
ADC1 and ADC2 prescaler
ADC3 and ADC4 prescaler
https://github.com/modm-io/cmsis-svd-stm32