net: cosmetic: Do not use magic values for ARP_HLEN

Commit 674bb249 ("net: cosmetic: Replace magic numbers in arp.c with
constants") introduced a nice define to replace the magic value 6 for
the ethernet hardware address. Replace more hardcoded instances of 6
which really reference the ARP_HLEN (iow the MAC/Hardware/Ethernet
address).

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>