Audit RTCPRE Shift and Mask Values

L0 - RTC Prescaler appears in RCC_CR, bit 20, 2 bits wide (RM0367 Rev 2)
L1 - RTC Prescaler appears in RCC_CR, bit 29, 2 bits wide (RM0038 Rev 10)
F0 - no prescaler setting (RM0091 Rev 7)
F1 - no prescaler setting (RM0041 Rev 4)
F2 - RTC Prescaler appears at RCC_CFGR, bit 16, 4 bits wide (RM0033 Rev 6)
F3 - no prescaler setting (RM0316 Rev 3)
F4 - RTC Prescaler appears at RCC_CFGR, bit 16, 4 bits wide (RM0090 Rev 8)
RM0033r5 implies the mask should be 0x1f.  (not sure that's the latest, but what ref man were you getting this from? (The 16 is definitely correct)