cpu/stm32/i2c: Fix error flag clearing in sr1
cpu/stm32/i2c: Fix error flag clearing in sr1

This commit fixes the clearing of a error condition after read.
This causes the incorrect errorcodes if the register is read
then an error occurs, then it is cleared.
By clearing only after the error is processed the bug is fixed.
This can be tested by reading a i2c slave that is not there.

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