stm32_common/i2c_2: fix unused *i2c when no DEVELHELP
stm32_common/i2c_2: fix unused *i2c when no DEVELHELP

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