shell/sc_can: fix uninitialized warning
shell/sc_can: fix uninitialized warning

When compiled for `hifive1` board with `gcc-7.2.0` this warning was raised:

'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

josephnoir/RIOT,authmillenon/RIOT,BytesGalore/RIOT,jasonatran/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,A-Paul/RIOT,kbumsik/RIOT,A-Paul/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,cladmi/RIOT,ant9000/RIOT,A-Paul/RIOT,miri64/RIOT,biboc/RIOT,cladmi/RIOT,x3ro/RIOT,biboc/RIOT,yogo1212/RIOT,authmillenon/RIOT,basilfx/RIOT,OlegHahm/RIOT,mfrey/RIOT,basilfx/RIOT,OTAkeys/RIOT,biboc/RIOT,smlng/RIOT,rfuentess/RIOT,mtausig/RIOT,kaspar030/RIOT,lazytech-org/RIOT,cladmi/RIOT,x3ro/RIOT,kbumsik/RIOT,toonst/RIOT,avmelnikoff/RIOT,basilfx/RIOT,avmelnikoff/RIOT,ant9000/RIOT,mfrey/RIOT,jasonatran/RIOT,OlegHahm/RIOT,A-Paul/RIOT,gebart/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,OlegHahm/RIOT,smlng/RIOT,rfuentess/RIOT,mtausig/RIOT,kaspar030/RIOT,mfrey/RIOT,lazytech-org/RIOT,yogo1212/RIOT,cladmi/RIOT,gebart/RIOT,RIOT-OS/RIOT,kbumsik/RIOT,mtausig/RIOT,aeneby/RIOT,josephnoir/RIOT,kYc0o/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,lazytech-org/RIOT,smlng/RIOT,mfrey/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,josephnoir/RIOT,biboc/RIOT,mtausig/RIOT,toonst/RIOT,BytesGalore/RIOT,BytesGalore/RIOT,authmillenon/RIOT,josephnoir/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,basilfx/RIOT,ant9000/RIOT,toonst/RIOT,BytesGalore/RIOT,avmelnikoff/RIOT,kYc0o/RIOT,ant9000/RIOT,toonst/RIOT,kbumsik/RIOT,kbumsik/RIOT,OlegHahm/RIOT,mtausig/RIOT,rfuentess/RIOT,x3ro/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,x3ro/RIOT,x3ro/RIOT,smlng/RIOT,yogo1212/RIOT,jasonatran/RIOT,OTAkeys/RIOT,mfrey/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,miri64/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,aeneby/RIOT,kaspar030/RIOT,jasonatran/RIOT,gebart/RIOT,rfuentess/RIOT,OTAkeys/RIOT,avmelnikoff/RIOT,cladmi/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,aeneby/RIOT,authmillenon/RIOT,miri64/RIOT,gebart/RIOT,aeneby/RIOT,josephnoir/RIOT,kaspar030/RIOT,aeneby/RIOT,toonst/RIOT,biboc/RIOT,gebart/RIOT