netfilter: log_arp: Use ARPHRD_ETHER instead of literal '1'

There is one macro ARPHRD_ETHER which defines the ethernet proto for ARP,
so we could use it instead of the literal number '1'.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>