netfilter: nf_log_syslog: Merge MAC header dumpers

The functions for IPv4 and IPv6 were almost identical apart from extra
SIT tunnel device handling in the latter.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>