gnrc_pktdump: remove some duplicate calls to od_hex_dump()
gnrc_pktdump: remove some duplicate calls to od_hex_dump()

`od_hex_dump()` is called if `hdr_len < pkt->size` to print the rest
after `hdr_len` of `pkt`. So if we just leave `hdr_len = 0` instead of
calling `od_hex_dump()` for every other NETTYPE, we achieve the same
effect.

As it is more effective (and already done in some cases) to re-set
`hdr_len` when the header was printed, we initialize `hdr_len` first
with 0 now.

OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,basilfx/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,basilfx/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,basilfx/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,basilfx/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT