drivers/cc1xxx_common: L2 netstats & cleanups
drivers/cc1xxx_common: L2 netstats & cleanups

- Added required logic to provide correct L2 netstats when the module
  netstats_l2 is used.
- Refactored code to use gnrc_netif_hdr_set_netif() over manually setting the
  pid to ease future refactoring.

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