net: arp: Add ethernet header if it is missing

If we are suppose to send an IPv4 buffer that does not have
ethernet header, then add it here so that the packet can
be sent properly.

Change-Id: If873bd299d7f2d6762a6e593ef69043a9190e976
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>