cpu/stm32_common: fix month encoding in RTC driver
cpu/stm32_common: fix month encoding in RTC driver

rfuentess/RIOT,toonst/RIOT,smlng/RIOT,jasonatran/RIOT,smlng/RIOT,authmillenon/RIOT,kaspar030/RIOT,basilfx/RIOT,yogo1212/RIOT,smlng/RIOT,smlng/RIOT,mtausig/RIOT,josephnoir/RIOT,ant9000/RIOT,authmillenon/RIOT,lazytech-org/RIOT,kYc0o/RIOT,mtausig/RIOT,cladmi/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,cladmi/RIOT,aeneby/RIOT,miri64/RIOT,mfrey/RIOT,OlegHahm/RIOT,josephnoir/RIOT,miri64/RIOT,aeneby/RIOT,josephnoir/RIOT,cladmi/RIOT,mfrey/RIOT,authmillenon/RIOT,mtausig/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,kaspar030/RIOT,josephnoir/RIOT,yogo1212/RIOT,aeneby/RIOT,toonst/RIOT,x3ro/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OlegHahm/RIOT,cladmi/RIOT,rfuentess/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,OlegHahm/RIOT,mfrey/RIOT,aeneby/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,toonst/RIOT,OTAkeys/RIOT,ant9000/RIOT,OlegHahm/RIOT,cladmi/RIOT,ant9000/RIOT,aeneby/RIOT,authmillenon/RIOT,mfrey/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,smlng/RIOT,ant9000/RIOT,kYc0o/RIOT,rfuentess/RIOT,kaspar030/RIOT,basilfx/RIOT,x3ro/RIOT,miri64/RIOT,toonst/RIOT,kYc0o/RIOT,mtausig/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,josephnoir/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,x3ro/RIOT,jasonatran/RIOT,lazytech-org/RIOT,lazytech-org/RIOT,toonst/RIOT,x3ro/RIOT,yogo1212/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,lazytech-org/RIOT,jasonatran/RIOT,basilfx/RIOT,mfrey/RIOT,mtausig/RIOT,x3ro/RIOT,basilfx/RIOT