ethernet_interface: Simplify hwaddr string
ethernet_interface: Simplify hwaddr string

Change-Id: I881f7e53712ebad4762ae34bf55cda8959a0fc97
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/phosphor-networkd,openbmc/phosphor-networkd