net: ethernet: Avoid overlapping memcpy for tx packets

This was reported by valgrind

Source and destination overlap in memcpy(0x80c18c0, 0x80c18c0, 6)
   at 0x4035C3E: memcpy (vg_replace_strmem.c:1023)
   by 0x80978CA: ethernet_send (ethernet.c:319)
   by 0x8061B9B: net_if_send_data (net_if.c:281)
   by 0x805F215: net_send_data (net_core.c:399)
   by 0x8080998: send_mldv2_raw (ipv6.c:2858)
   by 0x8080BA6: send_mldv2 (ipv6.c:2889)
   by 0x8080D6C: net_ipv6_mld_join (ipv6.c:2915)
   by 0x8061EF0: join_mcast_allnodes (net_if.c:438)
   by 0x8062CA4: net_if_ipv6_addr_add (net_if.c:826)
   by 0x8062296: net_if_start_dad (net_if.c:557)
   by 0x8066667: net_if_up (net_if.c:2107)
   by 0x8066AF0: net_if_post_init (net_if.c:2341)

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